Limit the number of users in groups?

Hello,

I am interested in a way to set the limit of users in a group to be just 1. Ideally we would like to be able to have our project assignment BP automatically populate our project shell groups. The problem becomes that while users get effectively added to the group, it does not overwrite, or take away users from the group.

We would like a way to either be stopped from adding more than one user, or to be able to see who is already in the group as a part of the assignment BP. Does anybody have experience with this? any information or ideas are appreciated.