Open Social has a 4th group type: Secret Group
It allows users to create groups, that cannot be found in the overviews/search unless you are a member of that group.
A site manager can still determine platform-wide if normal users may create these in the first place.
But for the communities that do allow this for normal users: we've made sure that site managers are always able to see all secret groups in the overviews!
Check out the video to see it in action!