I was just looking for this as well and am glad it has already been asked. We are starting to make more active use of groups, and need to keep an eye on a particular group right now to make sure they are starting to participate. Filtering the user list by group would be a huge help.
Is it now possible to search for users by group? I would be especially interested to have some sort of advanced search for users where I can ask: Who is in Group A and Group B. This would influence the way I would structure the groups within our community.