Making new friends

Like the title says I would like to make new friends. Im a male freshman and I already have a friend group from middle school, we are in the same high school. 2 of the people in my friend group became friends with this new girl at our school during this summer program that our school has. They are best friends after only knowing each other for 3 weeks.

Today is the first day we meet her but they don’t bother to introduce us. Also they added her to our group chat without telling us thats hella wrong for me and my friend who dont know the new girl because we talk about serious stuff in there. I cant bring anything up because i would be “overreacting” but it doesn’t sit right with me. Ik that friend groups change from middle to high school but i didn’t think it would happen so soon.

They still talk to us but i can tell that they’re drifting from us and will eventually stop talking to us. Before this happens I want to make new friends in my grade, so i had asked one of my male friends if he can help me. He said that the guys think im chill and dont care if i hang out with them, but i dont know how to jump into a friendship like that. I need to know smthn about the person or people first.

Do you guys have any advice on how i can be more open and outgoing so i can make more male friends?