Only one person of the 10 with whom you share can play at a time. This will not restrict you from playing though. It's not clear whether you can play a co-op game together on a single copy. To me this seems better than actually lending a disk to someone since you can still play your game while you lend it out.

I have no idea why Microsoft are saying outright you can't lend games, since you can share them. I think their messaging has been absolutely awful.