3
Maybe you can use one of RegKeys or Reg codes modules.
It is not exactly what you need and I never used any of them. Also I'm not shure they are in development any more. But at least you could have some ideas from them.
The way one work is, he generates a code, you give this code to a user and when user registers, it has to enter that code to register. I think there was an idea to be posible to move the user in some group, based on code he had.
If you try them, do it on a test instalation ...