To re itterate what has already been asked:

Do you want to watch, or participate?

Talking to her, while sober is the only way to go. If you want to do it as foreplay, in bed, great, but having a serious discussion afterwards, is required. Sometimes, finding a stranger is better than a friend, because, this way, no worries about rumors being spread, or him popping over for a repeat when you arent around. This was a mistake we made early on, and our friend thought because he had spent a night with us, that anytime he wanted he could grab a tit or ass, or proposition my wife. Not Cool.. that ended our friendship. And we learned from it.

If you are planning on taking an active role, setting up a threesome is rather easy, so long as you can you discuss what the ground rules are. whats allowed, whats not, and make sure these rules are what you both agree to.

Also, remember one final thought, sometimes fantasies are best left fantasies. Swinging, which is what you are considering on whatever level you both decide, Has been known to both improve relationships, as well as destroy them. Jealousy can pop up, as well as other issues.

Be prepared.