I think it IS official. Or that I am on the beta that's why I got the notification of family sharing.
Anyhow, you can't share your games like typical sharing.
For one, you can only share them with one at a time.
And for two, you will have to share your entire library at once. Not like one game to one and another to other. So you and the one with which you share can't play together.
Also, there is a limit of 10 with which you can share. Google steam family sharing. There's a complete FAQ on official site of steam.