With the list of members that's displayed when adding new members to a group, add an additional column that lists the groups they're already a member of.
It would be very useful to be able to have a 2nd list of people besides members. I am thinking of a potential member list, different than members, that can be communicated with to move them towards being members. Perhaps this is a separate request compared to Denise???
@Ken Witt: Would you mind explaining further why you'd like to have this ability and how you would use this?