Automatically add to secondary usergroup(s) upon registration?

[Cyricx]

I'm trying to figure out how I can automatcially add people to secondary usergroups when they register.

The reason for this is that I want them to be able to disable viewing for specific forums. The best way I've come up with to do that is to have the individual forums I want them to be able to disable to only be viewable by a specific usergroup.

When someone has been a member of a usergroup, they can leave and join it at their whim.

So, I'm just needing to figure out a way to add them to the secondary usergroups upon registration

Anyone know how to write this mod? or what code lines I might be needing to locate to hack it in myself?