mutually exclusive usergroups

[Sculli]

I am just going to try this myself. What I would like to do is to have mutually exclusive public usergroups. The way this would work is that if I user wants to join a public group, it will check whether that use is already part of some specified user groups, and if that is the case, then a message will appear stating that the user can not join that group because he or she is already in a different group and may not belong to two groups at the same time.

Which template do I need to look through for group membership joining.

I assume I can achieve what I want just using conditionals?

Thanks in advance for any hints you may be able to provide.