Dont think coaxing your gf into a orgy is easy. I had only 1 successful ex who was into this. The rest didnt even want to try asking, just hinting lead to a huge quarrel.
Firstly she is super open, and game for almost anything.
Secondly, she asked me back, what if its MMF and not FFM
Lastly, we tried a few times before we stick to the last "friend"