Application registration/locking

[somespam]

Hi
Can anyone recommend methods or articles on creating a "registrati on" system for a web-based application

I.e
Application gets installed on a web domain e.g. www.hello.com, the user accesses the app and (the first time) they are redirected to a registration page on www.ourapp.com. They enter basic details and this "unlocks" the app on hello.com. After that, the app checks (every day?) to see if registration is OK and only runs if it is

I'd just like to read up about methods of implementing this, security risks, etc. What code is required in the app? In the registration page? How can we, for example, limit the app to 5 users and embed that securely in the registration information so that it can't easily be hacked/overriden

Thanks for any help.