I've been adding some privacy controls and thinking about who can see a post and comment on it.
It struck me that 'groups' wasn't granular enough and that led me to the idea of having a list of 'friends'.

You choose to add someone to your friends list, then you can restrict a post to members on that list. It is a one to many xref table.

So the new privacy options are public, members only (any member in a group set to view_members_only_posts), friends only (members on your list), and private (only the author can see it, no comments).

There is a param for 'levels' of friends , this could allow some friends to be more in than others. Not sure I'll use it.