Yeah don't go up to her and ask!
Let you gf go over to her and chat & have a couple of drinks with her.
Then you go over and join them, cue more drinks.
After a while decide you are going and invite her back to your place for a drink.
If she says yes, she probably has an idea what's going on and is up for it.
Then at home after a couple mention that you have both always wanted to, but don't ask her to.
Watch her reaction. If reaction is ok, then ask her for her opinion on the subject?
Then just take it from there.
If she says no, make sure your attitude with her doesn't change though, just be nice and say bye
