Valve has started sending out beta invites for Steam’s Family Sharing program, so be sure to check your inboxes.
1,000 invites have been sent out to users who signed up for the beta. If you end up with one then all you have to do is follow these instructions. The service allows Steam users to share their game libraries with up to 10 users, each user is allowed to earn their own achievements and keep their own save progress.
It’s worth noting that a library can only be accessed by one user at a time and the owner of the account has priority access at all times.