After set up and integrate parse.com on your project the important thing is to sign up or can say create users.

Parse provides facility to their users to access their information in a secure manner.

ParseUser is the class that handles much of the functionality of the user. Basically ParseUser is the sub class of ParseObject.

ParseUser have some properties that that are
username : name of the user that it is requires. password: It is also required.
email: It is optional

Means If you are setting these field you dont need to use put() method.

Here is the code to simple signup functionality after set up integration in your project.

ParseUser newUser = new ParseUser(); 
newUser.setUsername(usernameString); 
newUser.setEmail(emailString); 

newUser.setPassword(passwordString); 
newUser.signUpInBackground(new SignUpCallback() { 
public void done(ParseException e) { 
if (e == null) { 
Toast.makeText(getApplicationContext(),"Sign up Successfully.",Toast.LENGTH_LONG).show(); 
} else { 
Toast.makeText(getApplicationContext(),"Error in sign up 
:"+e.getMessage(),Toast.LENGTH_LONG).show(); 
} 
}});