Private Registration

[Wired]

I need to install a forum for a software product that I develop.

What I would like to do is:

Require a password/code for registration to the forum
Only those with the registration code can signup
Disable the forum for guests

I have an automated software delivery system in place. If I were able to disable registrations, except to those that have a special code, then when a customer purchases my software, I can send the registration code to them with their order email. In turn, they could visit the registration page and enter their code when signing up and become a member. Without the code, new members could not signup.

Is this possible? I've searched through add-ons section and found one plugin that was close to this, but is no longer being supported.

Thanks for any suggestions