Hey guys, I want to swing a threesome with my girlfriend and this other girl - the setup is perfect, so I want to do it tonight without screwing up.

Setup: I had a party at my place the other night, and while it's in full swing my gf and I discreetly (I thought) sneak into my bedroom. We come out 15 minutes later and rejoin the party. I walk away to mess with the music, and this hb8 (roommate's friend) comes up to me and calls me out on disappearing with my gf. I laugh and joke about it, and she says "you should invite me next time". We joke around a bit more, talk about other stuff, I ask for her number and she enthusiastically gives it to me.

I told my gf about it and she's down (met the hb8 earlier and liked her), but obviously it's up to me to make it happen. How should I do this? Any general threesome advice is appreciated as well.