Modeling Groups with Admins, Notables, and Members

My app has Groups, and Groups have an Admin, Notables (people with
additional permissions), and Members (people with standard permissions).
I need help with modeling these, as I have lot's of ideas but don't
really know where to start, e.g. do I use a Memberships model? If so,
where would Notables and Admins fit in? Do I also need a permissions
model which handles the rights of Notables and Admins, too?

How would you model the relationships for this?