Just ran into this very problem, where a group’s membership is not administered by a member of the group.
Our use case
We want a handful of non-Discourse-admins to be able to manage group membership for several groups without becoming members of those groups themselves.
Extra credit
This is probably asking too much, but I thought I’d throw it in… it’d be awesome if a group could be named as admin of another group (i.e., membership of the @bar group can be managed by any member of the @foo group). This would allow us to have a “group managers” group containing our non-Discourse-admins in charge of managing other groups’ memberships. 