Happened with my ex who I (21M) was dating at the time (22F). We always considered the possibility of doing it but one day when my friend at the time (22M) was visiting from out of town I hadn’t seen in a while the three of us hung out in the living room drinking and playing some party games. Eventually after we were all a bit buzzed I noticed my friend taking some glances at my gf subtlety when she would walk away and he would look at her ass or when she would bend over and he would look at her cleavage. She was a very busty woman who got lots of attention in pretty much what she wore and he definitely noticed too
For some reason the idea sort of popped into my head so while she was in the other room I brought up the idea to her again and she said she was down so long as I was sure that was what I wanted to try doing. I said I was down so she went into the bedroom to get changed into something skimpier.