User Group Matrix

I am curious if anyone has the same issue I have and how they may have solved it.  My organization has various kinds of members: members, board members, officers, instructors of various types, etc.  I have forums that only certain members are allowed into.  The problem is that someone might be a member of more than one type.  In punbb you have to assign a single user group to someone instead of being a matrix where you assign multiple groups to a single person.

Any thoughts on this?