Easily Autoassign a new member to a user group

[FireFish]

I've had this idea for a while & wanted to share it; I'd LOVE to see someone code this:

We have 'hidden' forums reserved for members of a specific secondary user group.
However, to get them assigned to that group, I have a list of instructions I send them.
1) Register
2) Click 'Contact Us' & request for permissions promotion
3) Wait to be approved.

My Proposed Modification to solve this:
It would be neat to have a special field on the registration form called 'Invitation Code' or something like that, which, when entered, will automatically join that new user to that secondary private user group.

Alternatively, being able to send a special 'invitation link' which will automatically have that field filled in would be nice.

Obviously I'd want to be able to make multiple invitation codes which corespond to multiple user groups.

This concept is kind of like a Coupon Code in a shopping cart; you enter a predefined code & automatically receive a discount; no need for manual approval.

p.s. - the secondary user groups I am referring to here are NOT joinable. They are for BETA testing software & only selected BETA members get invited.

If anyone has any input on this idea, please reply to this thread.
I'd love to see it.