Hidden membership groups - should members be hidden from eachother, too?

This is a much desired feature for us, as any connection to our organization is by default categorized as sensitive information. As a volunteer organization we want to give some of users access to create and/or manage their own groups and let them add guest users to share emails and files with. However, membership in these groups does not entail the right to know the identity of the other members.

Today we work around this by using distribution lists, which is sub optimal as group owners can only add existing external contacts. without giving them too much admin access. These request are handled by central office by the system admin and two recipient admin.