Let me put it this way:
If your friends were going to be ‘stealing’ something anyway, and you weren’t going to steal yourself but just ‘hang with’ them. . .would you?
And if you were all ‘caught’ by the police, you would be charged with being an ‘accessory after the fact’.
Same with drinking–as a legal drinker, you probably would not be cited for illegal drinking, but if you were ‘caught’ with the underage drinkers, you would definitely get some kind of citation --even if it were as ‘minor’ as having their parents (and yours) be deeply, profoundly disappointed and betrayed by you for not being ‘responsible’ --because as the ‘legal adult’ you do know better than to ‘let people drink illegally’ (whether your friends ‘will do it anyway’ or not).
Do you want to be the kind of person who lets sin (and illegal drinking by those friends is sinful as it flouts the law–and they WILL, if caught, face a penalty) go by because, "hey, they’ll do it anyway, I’m not supplying it, I’m just hanging’?
By ‘hanging’ with them, you are giving them tacit approval. "It can’t be that bad–J doesn’t have a problem hanging with us even if we aren’t legal.’ You are cooperating with their sin and encouraging it if you ‘drink with them’ --and I think you know it already, and that’s why you’re asking.