Allow user to choose usergroup at registration

[rrr]

I thought I found the thread on how to do this at vbulletin.com, but it seemed to be missing the final step. I just found out that the reason it was missing the final step was that the standard vbulletin doesn't have a feature like this:

I'd like for my forums to have 2 usergroups (like 'buyers' and 'sellers').

At the registration point, I know I can add a new required field that asks if the person is a seller.

What I don't know is how to take that information (seller = yes) and have it automatically add that user to the 'seller' usergroup.

I'm hoping to have it as automated as possible.

Does a hack like this already exist?