While writing code to make a clear separation of working modules and codes you need to separate the blocks functions properties and classes so that understanding of the code is more effective

public interface INudgeService
    {
        #region "==== Operation Contract ===="

        /// <summary>
        /// To add user details into the database
        /// </summary>
        /// <param name="emp"></param>
        /// <returns>Return message,status and last inserted employee record</returns>
        [OperationContract]
        [WebInvoke(Method = "POST", ResponseFormat = WebMessageFormat.Json,
        BodyStyle = WebMessageBodyStyle.Bare,
        UriTemplate = "Register")]
        EmployeeResponse RegisterUser(EmployeeRequest emp);

        /// <summary>
        /// To authenticate user
        /// </summary>
        /// <param name="emp"></param>
        /// <returns>Return message,status and  employee record</returns>
        [OperationContract]
        [WebInvoke(Method = "POST", RequestFormat = WebMessageFormat.Json, ResponseFormat = WebMessageFormat.Json,
            BodyStyle = WebMessageBodyStyle.Bare,
            UriTemplate = "CheckLogin")]
        EmployeeResponse LoginUser(EmployeeRequest emp);

        /// <summary>
        ///To delete nudge of the user stored in the database
        /// </summary>
        /// <param name="accessToken"></param>
        /// <param name="Id"></param>
        /// <returns>Returns message status and nudge details</returns>
        [OperationContract]
        [WebInvoke(Method = "POST", RequestFormat = WebMessageFormat.Json, ResponseFormat = WebMessageFormat.Json,
        BodyStyle = WebMessageBodyStyle.Wrapped,
        UriTemplate = "Delete")]
        NudgeDetailResponse DeleteNudge(string accessToken, int Id);

        /// <summary>
        /// To show content/details of the user stored in the database
        /// </summary>
        /// <param name="accessToken"></param>
        /// <returns>Return message,status and  employee record</returns>
        [OperationContract]
        [WebInvoke(Method = "POST", RequestFormat = WebMessageFormat.Json, ResponseFormat = WebMessageFormat.Json,
        BodyStyle = WebMessageBodyStyle.Wrapped,
        UriTemplate = "UserProfile")]
        EmployeeResponse GetUserProfile(string accessToken);

        /// <summary>
        /// To send nudge details/messages of the user to the friends
        /// </summary>
        /// <param name="accessToken"></param>
        /// <returns>Return message,status and  employee record list</returns>
        [OperationContract]
        [WebInvoke(Method = "POST", RequestFormat = WebMessageFormat.Json, ResponseFormat = WebMessageFormat.Json,
        BodyStyle = WebMessageBodyStyle.Wrapped,
        UriTemplate = "NudgeFriends")]
        EmployeeListResponse GetFriendsToNudge(string accessToken);

        /// <summary>
        /// To change user details stored in the database
        /// </summary>
        /// <param name="emp"></param>
        /// <returns>Return message,status and  employee record</returns>
        [OperationContract]
        [WebInvoke(Method = "POST", RequestFormat = WebMessageFormat.Json, ResponseFormat = WebMessageFormat.Json,
        BodyStyle = WebMessageBodyStyle.Bare,
        UriTemplate = "UpdateUserProfile")]
        EmployeeResponse UpdateUserProfile(EmployeeRequest emp);

        /// <summary>
        /// To reset user password stored in the database
        /// </summary>
        /// <param name="Email"></param>
        /// <returns>Return message,status and  employee record</returns>
        [OperationContract]
        [WebInvoke(Method = "POST", RequestFormat = WebMessageFormat.Json, ResponseFormat = WebMessageFormat.Json,
        BodyStyle = WebMessageBodyStyle.Wrapped,
        UriTemplate = "ForgotPassword")]
        EmployeeResponse ForgotPassword(string Email);

        /// <summary>
        ///  To log out user
        /// </summary>
        /// <param name="accessToken"></param>
        /// <returns>Return message,status and  employee record</returns>
        [OperationContract]
        [WebInvoke(Method = "POST", RequestFormat = WebMessageFormat.Json, ResponseFormat = WebMessageFormat.Json,
        BodyStyle = WebMessageBodyStyle.Wrapped,
        UriTemplate = "LogOut")]
        EmployeeResponse LogOut(string accessToken);

        /// <summary>
        /// To find all the locations associated to the nudges
        /// </summary>
        /// <param name="accessToken"></param>
        /// <returns>Return message,status and  employee record</returns>
        [OperationContract]
        [WebInvoke(Method = "POST", RequestFormat = WebMessageFormat.Json, ResponseFormat = WebMessageFormat.Json,
        BodyStyle = WebMessageBodyStyle.Wrapped,
        UriTemplate = "GetNudgeLocation")]
        LocationListResponse GetNudgeLocation(string accessToken);

        /// <summary>
        /// To send and display the nudge send by someone to the particular user
        /// </summary>
        /// <param name="accessToken"></param>
        /// <returns>Return message,status and  nudge details</returns>
        [OperationContract]
        [WebInvoke(Method = "POST", RequestFormat = WebMessageFormat.Json, ResponseFormat = WebMessageFormat.Json,
        BodyStyle = WebMessageBodyStyle.Wrapped,
        UriTemplate = "GetEmployeeNudge")]
        NudgeListDetailsResponse GetEmployeeNudge(string accessToken);

        /// <summary>
        ///  To save nudge of the user for their friends
        /// </summary>
        /// <param name="nudgeRequest"></param>
        /// <returns>Return message,status and  nudge list details</returns>
        [OperationContract]
        [WebInvoke(Method = "POST", RequestFormat = WebMessageFormat.Json, ResponseFormat = WebMessageFormat.Json,
        BodyStyle = WebMessageBodyStyle.Bare,
        UriTemplate = "SaveNudge")]
        NudgeDetailResponse SaveNudgeForFriend(NudgeListRequest nudgeRequest);

        /// <summary>
        /// To get all the details of the company for creating nudges that is stored into the database 
        /// </summary>
        /// <returns>Return message,status and  company details</returns>
        [OperationContract]
        [WebInvoke(Method = "GET", RequestFormat = WebMessageFormat.Json, ResponseFormat = WebMessageFormat.Json,
        BodyStyle = WebMessageBodyStyle.Bare,
        UriTemplate = "GetCompany")]
        CompanyResponse GetCompany();

        /// <summary>
        /// To get all the roles associated with the users that is stored into the database 
        /// </summary>
        /// <returns>Return message,status and  role details</returns>
        [OperationContract]
        [WebInvoke(Method = "GET", RequestFormat = WebMessageFormat.Json, ResponseFormat = WebMessageFormat.Json,
        BodyStyle = WebMessageBodyStyle.Bare,
        UriTemplate = "GetRoles")]
        JobRoleResponse GetJobRoles();
    }

    #endregion

    #region "==== Employee Model ===="
    /// <summary>
    /// To pass as request for employee operations 
    /// </summary>
    [DataContract]
    public class EmployeeRequest
    {
        [DataMember]
        public int Id { get; set; }
        [DataMember]
        public string AccessToken { get; set; }
        [DataMember]
        public string CompanyId { get; set; }
        [DataMember]
        public string Company { get; set; }
        [DataMember]
        public string DeviceType { get; set; }
        [DataMember]
        public string DeviceUUID { get; set; }
        [DataMember]
        public string Email { get; set; }
        [DataMember]
        public string FirstName { get; set; }
        [DataMember]
        public string JobRoleID { get; set; }
        [DataMember]
        public string LastName { get; set; }
        [DataMember]
        public string FullName { get; set; }
        [DataMember]
        public string Password { get; set; }
        [DataMember]
        public bool IsDeleted { get; set; }
        [DataMember]
        public string ProfileImage { get; set; }
        [DataMember]
        public byte[] ProfileImageByteArray { get; set; }
    }

    /// <summary>
    /// To pass as list for multiple employees
    /// </summary>
    [DataContract]
    public class EmployeeList
    {
        [DataMember]
        public int ID { get; set; }
        [DataMember]
        public string FullName { get; set; }
    }

    /// <summary>
    /// For single employee response
    /// </summary>
    [DataContract]
    public class EmployeeResponse
    {
        [DataMember]
        public string Message { get; set; }
        [DataMember]
        public bool Status { get; set; }
        [DataMember]
        public string accessToken { get; set; }
        [DataMember]
        public EmployeeRequest data { get; set; }
    }

    /// <summary>
    /// For multiple employee response
    /// </summary>
    [DataContract]
    public class EmployeeListResponse
    {
        [DataMember]
        public string Message { get; set; }
        [DataMember]
        public bool Status { get; set; }
        [DataMember]
        public List<EmployeeList> employeelist { get; set; }
    }
    #endregion

    #region "==== Nudge Model ===="
    /// <summary>
    ///  Send request while manipulating with Nudge
    /// </summary>
    [DataContract]
    public class NudgeListRequest
    {
        [DataMember]
        public string Message { get; set; }
        [DataMember]
        public int TargetUserID { get; set; }
        [DataMember]
        public string AccessToken { get; set; }
        [DataMember]
        public int DaysTillExpire { get; set; }
        [DataMember]
        public int HoursTillExpire { get; set; }
        [DataMember]
        public int LocationID { get; set; }
        [DataMember]
        public string CreatedBy { get; set; }
        [DataMember]
        public string CreatedDate { get; set; }
        [DataMember]
        public int ID { get; set; }
        [DataMember]
        public string Location { get; set; }
        [DataMember]
        public string TargetUser { get; set; }
        [DataMember]
        public string Company { get; set; }

    }

    /// <summary>
    ///  For single nudge
    /// </summary>
    [DataContract]
    public class NudgeDetailResponse
    {
        [DataMember]
        public string Message { get; set; }
        [DataMember]
        public bool Status { get; set; }
        [DataMember]
        public NudgeListRequest data { get; set; }
    }

    /// <summary>
    /// For multiple nudge
    /// </summary>
    [DataContract]
    public class NudgeListDetailsResponse
    {
        [DataMember]
        public string Message { get; set; }
        [DataMember]
        public bool Status { get; set; }
        [DataMember]
        public List<NudgeListRequest> data { get; set; }
    }
    #endregion

    #region "==== Location Model ===="
    /// <summary>
    /// For creating location request
    /// </summary>
    public class LocationDetails
    {
        [DataMember()]
        public int ID { get; set; }
        [DataMember()]
        public string CompanyID { get; set; }
        [DataMember()]
        public string Company { get; set; }
        [DataMember()]
        public string Location { get; set; }
        [DataMember()]
        public string BeaconUUID { get; set; }
        [DataMember()]
        public string BeaconMajor { get; set; }
        [DataMember()]
        public string BeaconMinor { get; set; }
        [DataMember()]
        public string AccessToken { get; set; }
    }

    /// <summary>
    /// To pass as list for multiple locations
    /// </summary>
    [DataContract]
    public class LocationList
    {
        [DataMember]
        public int ID { get; set; }
        [DataMember]
        public string Location { get; set; }
    }

    /// <summary>
    /// For providing location response
    /// </summary>
    [DataContract]
    public class LocationListResponse
    {
        [DataMember]
        public string Message { get; set; }
        [DataMember]
        public bool status { get; set; }
        [DataMember]
        public List<LocationList> data { get; set; }
    }
    #endregion

    #region "==== Company Model ===="
    /// <summary>
    /// For passing company as list in response
    /// </summary>
    [DataContract()]
    public class CompanyList
    {
        [DataMember()]
        public string Company { get; set; }
        [DataMember]
        public int ID { get; set; }
    }

    /// <summary>
    /// For providing company response
    /// </summary>
    [DataContract()]
    public class CompanyResponse
    {
        [DataMember()]
        public List<CompanyList> data { get; set; }
        [DataMember]
        public bool status { get; set; }
        [DataMember()]
        public string Message { get; set; }
    }
    #endregion

    #region "==== Job Model ===="
    /// <summary>
    /// For passing joblist as list in response
    /// </summary>
    [DataContract()]
    public class JobList
    {
        [DataMember]
        public int ID { get; set; }
        [DataMember()]
        public string JobRole { get; set; }
    }

    /// <summary>
    /// For providing job response
    /// </summary>
    [DataContract()]
    public class JobRoleResponse
    {
        [DataMember()]
        public List<JobList> data { get; set; }
        [DataMember]
        public bool status { get; set; }
        [DataMember()]
        public string Message { get; set; }
    }
    #endregion
}

You can make the code easy by minimizing everything inside regions

Now your code looks like this

    #region "==== Job Model ===="
    /// <summary>
    /// For passing joblist as list in response
    /// </summary>
    [DataContract()]
    public class JobList
    {
        [DataMember]
        public int ID { get; set; }
        [DataMember()]
        public string JobRole { get; set; }
    }

    /// <summary>
    /// For providing job response
    /// </summary>
    [DataContract()]
    public class JobRoleResponse
    {
        [DataMember()]
        public List<JobList> data { get; set; }
        [DataMember]
        public bool status { get; set; }
        [DataMember()]
        public string Message { get; set; }
    }
    #endregion

Which is more easy to understand and make separation between the methods and properties