CodeCanyon - Registration System with Invitations | 1,89 MB
With this system you can protect your php pages in few minutes. How to do that, it's very simple, when a visitor wants to see your protected page, will need to login, but for registration on website, visitor need to be invited. The invite system is protected and the invitations can't be cracked.
DEMO:
http://codecanyon.net/item/registration-system-with-invitations/603208