Alrighty so everybody who joined the Steam Family Sharing beta group can now use the beta, with that in mind this is what we can do, should you choose to share yours games (and why the hell not):

1. Make a temporary password for Steam.
2. Give your username and temporary password to the person you want to share games with.
3. Check your email for the authorisation code you'll be sent to get them past Steam Guard.
4. When they log in, they need to go to settings>manage family sharing and devices.
5. They give their device a name and authorise it.
6. Ta-da! They can now go back to their profile and play your games, or set it up the other way around too.
7. Change your password back so now your account is safe and sound.

P.S. Protip: you can play each others games at the same time BUT only if you are playing *each others*. As in, Jack can play Jill's game while Jill is playing Jack's game, but if Jack decides to play one of his own games, Jill gets the kick.