I want group sex with my girlfriend. i want swapping another couple how to convence my girlfriend for swapping.

Thanks for asking: I'll let others decide whether this is an appropriate question for the "health tap" team. I don't know the answer, but I do know that the vast majority of women won't be receptive to a "swinger" / "group sex" lifestyle. You're young & it sounds like you want more time before you settle into a monogamous relationship, if you ever do. Be a gentleman & if she says "no", think of going separate ways.