I think you care too much about what other people think. I also understand this so don't see it as 'abnormal' but do think it indicates anxiety of some sort and that you could usefully seek help for that.
I think you somehow need to let go of feeling responsible for other people's behaviour and this sense that they are an extension of you.
They are who they are. Other people can see them for who they are. They know that 'you are not them'; you cannot control them and you are not the same as them, now or ever. They are just people you used to socialise with, because you had one or two things in common. They have other characteristics and behaviours that were never in common with you and yours.
Also, these people have met each other before, at previous parties, so that horse has bolted really. The new friends know about the 'old friends' and they still come.
Most people have different groups of friends like this; the student friends, work friends, hobby friends, parent friends etc and they can be quite different and have different values and lifestyles. Especially if the different groups hang about as groups, using the party as an opportunity for a catch up, that's fine. I do get your discomfort about the weed. For people who have no history of drug use, that can seem a big deal, even though, legally, societally etc it really isn't in the same category as other drug use.
The other thing is, do any of the people treat, respond to and 'present' you differently, in a way that makes you uncomfortable? Saying 'you're so...' when you know your newer friends don't see you that way, because you've matured, or just that you know them through different activities?