My girlfriends friends don't like me?

my girlfriends friends always seem to put my gf in a position where she has to choose between me or them and since they've known her since hs they have a way of getting into her head! they were having a barbecue at her friends house and my gf called and asked if i could come and they said i don't think he should come because some of her friends smoke weed and one of them has just got out of jail because my father is a cop and im training to be one right now and i don't have a problem with weed and stuff i use to do it in hs to but i grew up im 24 now and im trying to be cop and they don't like that so what do i do her friends don't like me because im gonna "narc" on them even though im not an officer yet