Last week, Valve introduced some changes to the Steam Family Sharing program which was announced late last year and currently in beta. While the core concept of sharing digital games with friends and family still applies, Valve has taken steps to beef up security by implementing a two-step validation process.
According to a post on Steam's community site, you must now identify the Steam users who may access and play your shared games, in addition to authorizing the actual computer. Prior to this, you would only have to authorize a machine to share your library of games, meaning any user who logged into their account on that computer could access your games.
"This allows lenders more control while reducing the risk of VAC or other bans resulting from an unknown user accessing and abusing shared games on an authorized machine," Valve reasons.
In its current form, Family Sharing allows you to authorize up to 10 Steam accounts on up to 10 machines to access and play your shared games. Users mays till request access to your shared library by sending you a request from any machine. You can read up more about this program on the official FAQ, which has been updated to reflect these changes.