My boyfriend’s M30 ex is friends with all of his friends but I’m F27 having a hard time befriending them. How can I get to know them?

My boyfriend M30 and I F27 have been together for about 5 months. His past relationship, or situationship had some communication issues. He thought that they were in a relationship, but she did not and was seeing other people the entire time. (They went on trips together, were intimate, met each other’s parents, talked about marriage, etc.) They met through a girl friend of his.

I’ve been trying to get to know his girl friends more, they all seem like great people that I would have a lot in common with. However, it feels like a lot of them don’t like me and like I haven’t had a fair chance to get to know them. I’ve noticed that his “ex”-whatever is friends with these girls and it makes me feel bad, like she’s some sort of ghost haunting us. It makes me wonder if they don’t like me because of her or something.

I don’t even know her. He barely talks about her and it’s not consuming our relationship or anything. I’ve already talked to him about it a little bit. When I’m with him and/or them, he’s confused about their reactions too. They’re not rude, just kind of passive or dismissive. I’m a pretty confident person and know that not everyone has to like me. But if these people are important to him, I want to at least know who they are. How can I go about getting to know them better despite all of this?