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.
Coming Soon: Players who share computers can also share their Steam games
September 11, 2013 - Steam Family Sharing, a new service feature that allows close friends and family members to share their libraries of Steam games, is coming to Steam, a leading platform for the delivery and management of PC, Mac, and Linux games and software. The feature will become available next week, in limited beta on Steam.
Steam Family Sharing is designed for close friends and family members to play one another's Steam games while each earning their own Steam achievements and storing their own saves and application data to the Steam cloud. It's all enabled by authorizing a shared computer.
"Our customers have expressed a desire to share their digital games among friends and family members, just as current retail games, books, DVDs, and other physical media can be shared," explained Anna Sweet of Valve. "Family Sharing was created in direct response to these user requests."
Once a device is authorized, the lender's library of Steam games becomes available for others on the machine to access, download, and play. Though simultaneous usage of an account's library is not allowed, the lender may always access and play his games at any time. If he decides to start playing when a friend is borrowing one of his games, the friend will be given a few minutes to either purchase the game or quit playing.
For more information about Steam Family Sharing and the beta program, please visit http://store.steampowered.com/sharing/.