Kicking a user is used to remove an user from a session. This is primarily used to remove someone when they have asked the game creator to do so.
Removing a user just to make room for a friend is not acceptable. If you would like to have a friend join a full session, message the other users in the session to see if they wouldn't mind leaving.
To kick a user, go to the game session and click "kick" on the user you want to remove. You must be the session creator in order to be able to kick someone.