Steam family sharing system now open to all28 Feb 2014
As the post states, “Players who share computers can now also share their available libraries with one another, each earning their own achievements and saving their own game progress to the Steam Cloud.” In order to enable access to your account on a shared computer, it’s necessary to have Steam Guard enabled under settings and opt-in under the Family Library Sharing settings.
Up to five different accounts can have access to your game library, as long as they’re on an authorised computer. Shared libraries can only be accessed by one user at a time.
Further instructions and information can be found at Valve’s Steam family sharing FAQ over here.