: (only one person in the circle of 'family' can play a purchased game at a
: time, unless the information has changed since I last read about it)

Microsoft says:

Give your family access to your entire games library anytime, anywhere: Xbox One will enable new forms of access for families. 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.

You (the owner) can play your game at any time, plus one member of your 'family'.

I don't know if it means that only one other person can access the library at once, or if only one other person can play a particular title at once. The former would still be pretty great, but the latter would be even more awesome.