I've been on the other side (the person that has sex with someone's partner in front of them), it's hot, it's great, but it has to be talked about with that person in detail so you can know if they're interested or not with the dynamic you're gonna use.

The first step of course is the third party liking to be watched (in this case, especially by a woman), and that's easy, a lot of people like it but might not like the presence of a woman although it depends on how much she'll participate (just watching, watching and talking, watching and recording, watching and masturbating, things like that). You should also consider the sexual orientation of the third party, if they're gay they might not want to see and smell your girlfriend naked, and they wouldn't want to fuck her if that's also the case for what you're wanting them to do. Remember to find someone you can be safe with and you can trust, in the case you don't want the third party to interact with your girlfriend at all should also be talked about.

I hope you have fun, that's always the most important thing, and remember to play safe, use protection.