The only limitation, it seems, is that only one person can be playing the shared copy of a single game at any given time.
Up to ten members of your family can log in and play from your shared games library on any Xbox One. Just like today, a family member can play your copy of Forza Motorsport at a friend’s house. Only now, they will see not just Forza, but all of your shared games. You can always play your games, and any one of your family members can be playing from your shared library at a given time.
So basically if my buddy 500 miles/kilometers away is on my share list, we could split the cost of the next great coop game?!?!?!?