Me(21M) and my girlfriend (21F) of 10 months, a couple weeks ago talked about having a threeway and I was quite surprised she agreed to it. We talked about it alot and trying to figure out who we should ask. We came to a conclusion on who to ask and it's one of her friends and she wants me to ask her friend (20F). I don't know how to ask her friend because I never talked to her at all or even interacted with her other than seeing her dance when me and my girl go linedancing. Any advice on how to approach this?

Also most I know about the girl is that she had her eyes on me for a couple months before she realized me and my girl are together.