Steam Family Sharing will enter a limited beta next week; members can express interest in the beta by joining the Family Sharing group.
Shared libraries may only be accessed by one user at a time, though the owner of the library will be given priority to access it at all times. If someone else is playing a game in that library, the friend will be granted a few minutes to either buy the game or quit out.
Games that require a third-party key, account or subscription will not be eligible to share with Steam Family Sharing. Sharing includes access to the library owner's DLC, but friends will not be able to purchase DLC if they don't own the base game as well. Region restrictions on games remain in place.
Read through the Steam Family Sharing FAQs here, and start making some more friends stat.