User Roles & private posts: View, but not edit

I have been wondering about this issue for a long time now. Is it possible to assign a role to a user, that allows them to view private posts but not edit them? I would like to allow friends of mine, to view posts that I don’t want to share with the public. On the other hand, I really would rather not give them the permission to edit anything there.

A second question: Does anyone know of a way, how to make WordPress behave similar to how Livejournal works with roles? In that users may be assigned to certain (ideally custom) groups and for each post one may specify the visibility: public, private or only visible to users in group “foo” and “bar”.

Thanks much!