The limitation, according to the official notice is that “a shared library may only be accessed by one user at a time. As the lender, you may always access and play your games at any time. If you decide to start playing when a friend is already playing one of your games, he/she will be given a few minutes to either purchase the game or quit playing.”
The Sharing program is currently in beta. "To express interest in beta participation, join the Family Sharing Group on the Steam community," states Valve, "The Family Sharing beta will begin in mid-September, when a thousand Steam accounts from this group will be granted access to share their Steam libraries. You’ll know you’ve been selected when you receive an email from Steam inviting you to try out the new feature."
More info can be found on the official Steampowered "Sharing" page.