How do I deal with a new girl in town?

Our roommate is getting a little more serious with the girl he's dating. How do I deal with her coming over all the time and throwing off our schedule/space etc. I guess I'm territorial lol but she's a great girl we've met a few times its just weird seeing as he only lives with us for a measly $100 to help him out so its our house. Anyway I'm being a brat right? I just don't know how to work with it all.