My husband always jokes about having another girl in the bedroom but remember if both parties are not 100% into this it could be a breaker in your relationship. You have to consider scenarios of him liking this other girl and if he's open to you allowing another girl in the bedroom... will he take it too far and cheat? I know alot of couples that were swingers.... and a few of those ended in divorce because one of the parties started seeing the other person on the sly. If you're going to do it invite a girl that neither of you know personally and will not talk to again so it because simply a played out sexual fantasy and nothing more. I could never personally share a person I love with someone else... if you are TRULY ok with this... and not simply to avoid him breaking up with you then power to you.
You can set some ground rules like... he cannot have vaginal sex with her or kiss her etc....