First let me say this....lucky bastard
Now thats out of the way, I dont think any amount of reassuring will convince her otherwise, it's probably her own insecurity stopping this whole thing so this situation has to be eased into slowly. How well acquainted is your gf and this other girl? If there isnt any kind of rapport between them I can see why she'd get jealous and treat her as the "other" girl.
I cant think of a quick fix that'd be...ethical....alcohol does crazy things if you wanted to try and go that route but she could end up feeling guilty for doing something she didnt want to do yet and that could keep it from being a reoccuring thing.
I think this is just gonna take time man, let her know you still love her and that your relationship with this girl is only what it is right now. If your patient and guide her into this slowly, you can have a hell of a situation for yourself
Have these two hang out at the party and let them get to know each other well, since it's her decision she's going to need that reassurance from the other woman that it's only a sexual thing and that she only wants to be friends.
Once your girl comes to terms with that and you have your first threesome, she'll realize how small of a deal it is and it's game on then. But I wouldnt put a timeline with this, I can understand this is a weird new experience for her and trying to rush it in any way could hurt your chances.