We can use Bakery SSO module for single sign on in drupal 7. It works with portal or master site and sub-sites or slaves sites, it provides the following features:-
1) If login into the portal site and navigate to sub-site, user will be auto-login.
2) If any user login into any of the sub-site and navigate to the portal site, user will stay loggedIn in portal site.
3) This module sync the common fields of the user table i.e email, password, status, picture etc.
4) For logout at one place and reflect to other sub-site or portal site, you have to share session table. You have to add $cookie_domain in your both portal site as well as sub-site's configuration file.
You can download this module easily from https://drupal.org/project/bakery.