In Starcraft II you can have it flip through your Facebook friends and add people that have the game to your Battle.net friends list. How do you do this in Diablo III?
I'm guessing that the above recommendation to add them in Starcraft II is the best way, if you have it that is. Because I'm pretty sure it's pulling the friends from their Bttle.net account and not from Starcraft II specifically so if they're on Battle.net, for Diablo III or otherwise, this will work.
My Starcraft II picked up my brother who only created a Battle.net account for a Diablo III guest pass and doesn't own Starcraft II.