When you left-click on the , a menu will pop up.  This menu contains a list of users to which you can switch, as well as the option to simply go to the login screen.
To either log in as another user, or to resume another user's session, simply click on that user's name.  If the user is already logged in, then you will switch to that user's session and you will have to unlock the screen.  If the user hasn't logged in, then you will be sent to the login screen.  Simply enter the user's name and password as requested, just as you do to log in normally.
When you switch to another user, your session will be locked.  You will have to enter your password to finish resuming your session.
