Sounds like an invite to at least have a discussion about sex, likes/turn on's, fantasies. How she feel about a 3 way with you and another guy? How do you feel about it? Is she interested in other women? etc etc. ... The best sex and best relationships start with communication and honesty. Good luck.

That sounds like the sort of situation that many guys would fantasise over and wish would happen to them.

Any luck yet, Pumped?

Ask your girlfriend if she might actually be interested in doing anything sexual with her best friend, or if she at least might like making out with her. Say that seeing that, and then letting her watch the two of you have sex would be like a dream come true. Tell her you love the thought of someone she's so close to watching the two of you making love. Make it clear you're not asking for anything to happen between you and her friend.

This is just what I would suggest. If she's willing to do that, you shouldn't necessarily expect more to happen (it's still much better than nothing), but there's a chance it will in the heat of the moment. Them at least making out first will make your girlfriend more likely to think of involving her friend in some way. Don't make that suggestion yourself, though.