Usergroups enabled by key-checking?

[CarCdr]

From what you have said thus far, this is very similar to the way forum passwords are managed. I would copy/adapt the code for forum passwords.

A couple of questions:
1) If I have entered a valid CD key, do I get access to just one forum or to multiple forums?
2) Is the forum(s) I get access to dependent on the key or does everyone with a valid key get the same access?

The answers to these questions will dictate your choices for where the passwords are stored in your DB.