Usergroups enabled by key-checking?

[Xenon]

Personally i'd use Colin's suggestion.

If you have you Key within the same DB as vB, it would just require a single custom profile field and a cron job.
Selecting just those users which are in the normal usergroup but have entered a key shouldn't be to server intense. run the script once a day should be enough.

The other way would be to edit profile.php so once the custom field is filled, it has to check the key table, and then modifies the usergroup of the user.