mutually exclusive usergroups

[Sculli]

It would be great if one could specify mutually exclusive usergroups.
Meaning if someone is in groupA that person can not be in groupB as well.
This would probably have to somehow work through child-usergroups, so that it's clear which groups are mutually exclusive to which groups.

[Sculli]

I thought about this a little more, and it can probably be done with simple conditionals. Like if someone wishes to join a particular usergoup, the machine checks whether the user is already a member of another usergroup id that is considered mutually exclusive.

To which template would I have to add such conditionals?