Invitation codes for registration

Membership Plugin 1.0 on WPMU 3.0.1

A project I am working on for a gym that has local users in its physical location, and is expanding to include selling memberships to access online-only content.

Existing members of the gym are to be granted access to the protected content, while others will purchase a membership. So, essentially, one paid membership and one free membership.

The issue is integrating the gym users into the Membership plugin who would be able to register themselves on the site.

Ideally, we would be able to implement some sort of activation code.

Has anyone implemented anything like this? Short of importing users (klunky), implementing a password-protected page on which the registration form would reside is the only viable idea I've come up with. EDIT: The problem with the later is that I can't figure out a to have a password-protected form and still have the pay-to-join form available without the password.

Any ideas would be greatly appreciated!

Thanks!
Casey