registering into different groups

[Oasis]

Hi, is there a way I can modify the form so that when a user signs up it signs up into a specific group? And I don't mean a default group. I mean that they can choose to enter a group. Is it possible?

If it is, could someone give me some hints on how, because I am trying to do a special offer thing, so that users with the correct promotion code can sign up into a group with higher disk quota. Butr first I need to know if it is possible to sign up into different groups...

any help would be appreciatd, thanks in advance.