My girlfriend (F20) and I (M21) are in college and have been dating for a year. We both have high sex drives, but I believe mine is a bit higher. We have sex, be it PIV, oral or hand stuff, almost every chance we get, amidst the chaos of college life and work.

We're planning to go on a short trip with other friends and couples to a nearby city when the next semester begins. We do this every sem to have some fun together, drink, chill and of course fuck with our respective partners.

This time, I want to expand our sex life a little and let one of our friends (F20, bisexual) join us during sex. I don't want this to be a proper threesome, but only let the other girl fondle my balls, lick my gf's clit, maybe massage my prostate while me and my gf have sex.

This friend has previously expressed desire to watch us have sex, and she has been friends with both of us separately since before we met.

My problem is how to communicate this to my girlfriend. She is fairly open sexually, but is conservative when it comes to such things like multiple partners or other participants in sex. She has trust issues and is very insecure about loyalty, though we've had a strong relationship since the start.

(I believe it won't be a problem proposing this idea to this friend of ours, because she is bi, single and very open. I have even said yes when she jokingly asked to watch us fuck before, and she seems enthusiastic but we never took it seriously)

TLDR: how do I propose a semi-threesome with mutual friend to my insecure gf?