The tricky part is that those ad-hoc groups may need special parameters or data. But that can be solved by improvements to parametric roles and/or archetypes.
TODO: contacts: family, friends
TODO: invitesMidPoint is good to maintain formal relationships, such as owner or manager. Those are used a lot in enterprises and they are essentially the foundation of any organizational identity management. However, there are also informal and self-managed relationship between users, such as the relation of "collaborator", "friend" or "family member". Those relations may be essential for future identity management, e.g. ad-hoc collaboration on a project, cooperation on non-business activities, parents paying bills for children and so on.
The users should be able to set up those relationships with existing users, invite new users and so on.
MidPoint should allow to set up and manage those relationship. This is not that unlike the way how social networks work. However, the goal is quite different. We do not want to entertain the users and sell them ads. We want to use those relationships to allow real cooperation.
Note: there is a data protection aspect to consider.
Smart identity merge/split