How do you approach people who are already in a circle?

I am at a time in my life where I really only know a few people and even then, I don’t get along with them nearly as well as I used to. I’m trying to get to know more people; however, nobody really seems interested; they’ve all got their own friends and friend circles and I don’t really know what I can do for myself in a situation like this. So, how would I make friends in a situation like this? How should I approach people who’ve got their own friend group?

If it matters, I’m an 18 yo guy.