Bf wont introduce me to his friends?

Things have been going great with my boyfriend of 3 months, but I feel bad because whenever we hang out with other people its always with my original friend group. He has melted into my group of friends pretty well-- but I can't help but feel that I should be making some effort to meet his other friends. About once a month he'll hang out with some people from the college swim team but I'm never invited. There are swim girls at the parties and I know some of the swim guys bring along their girl friends.... So why am I not invited? Is he embarrassed of me? Is he embarrassed of his friends? I don't think its a cheating issue-- he would never do that. He doesn't try to hide the fact he's going out with friends either so what should I do? I kind of want to talk to him about it but I don't want to come across as over protective. I understand he needs some space with his guy friends but I'm just a little offended over the fact that there is other girls at the parties too....