Lists: Add friends/users + collab. lists

An awesome feature would be to create and edit lists with your friends, family or partners.
I comprehend that for this to happen, users need to be able to add each other as a friend.
In the case the idea could be executed, creating a list would have some extra settings for you to give the permissions needed for other users to modify, for example, "list owner"; "admin" and "collaborator".