"Registration by invitation only" feature more improved. Because the user can type in his browser directly ANY ID, even if does'nt exists he can join the site, and this makes it useless. whatever ID you type in your browser you can still join. Test and try something like http://www.mysite.com/?idFriend=99999 . That means the user with id 99999 invited you. You can type even 999999999, and you can still join, even if user with id 999999 doesn't exist.