Just tell her there's nothing wrong with making new friends. Get her a little drunk then when you think she's picked out a guy tell him that you're cool with everyone going back to a room and that he needs to bring it up to her while you're all together.
She'll be turned on and freaked out all at once. If you have a room already it will be easier. Then you and the guy just keep pushing to go back to the room to have some more drinks and there you have it.
