Preliminary look: The use of Relationship sounds kind of similar to the proposal for a Member or Membership entity to track properties of a Group's members. It's possible for something to be both a Member(ship) and a Relationship.

We want to support roles and permissions beyond just a hardcoded notion of "admin", "moderator", and so on, but we want to also cover a mapping from these concepts to the roughly equivalent permissions you might expect them to have (but with the recognition that different people have different expectations for this).