Well, technically this is still beta, so I'm wondering if maybe they'll be tweaking it to allow multiple users playing different games on the same library simultaneously. (Although I'm not going to hold my breath.) One way off the top of my head is maybe just checking the ip address of the two computers trying to access the library at the same time and if they are on the same internal network (in your home or whatnot) they can play concurrently. That way it can be shared in the household but not just a pool of games for your 10 friends across the world.
That said, this is cool for letting your siblings or kids play while you're at work or something or trying to get your friends hooked on a game (so they'll buy a copy to play with you) when the demo (or lack of one) doesn't cut it. I hope they also allow some personal settings tweaking for it. I'd like to set an automatic time frame when the library will be accessible to family and automatically kick them off (after warnings of course). That way I can let them play when I'm at work and boot them before I get home so I can hop on right away.