Relationship advice?

I understand that my boyfriend needs alone time his friends, but he doesn't seem to bring me along with him often enough. He'll usually tell me "I'm gonna go hang out with so-and-so." and then leave, even if it's someone I've never met before or haven't heard of. And a few days ago he told me he's going to a party with one of his friends, and it really bothered me that he didn't even ask if I'd be interested in going with him. It wasn't an exclusive event or anything like that either, just a really small and casual house party with mostly strangers. 
Am I overreacting? Or does stuff like this bother you too?