Until now, to share your Steam Library with someone all you had to do was authorize another computer. Now Valve has implemented a change that will require you to not only authorize the computer but the other person has to have an authorized Steam account as well. You can share your library on 10 computer through 10 accounts. Valve is implementing this measure just in case someone wants to abuse your library and get banned, this way your account won’t be the affected.
The feature will be supported on PC, Mac, and Linux and is currently in beta.