What the hell? Stupid girls! Rooming situation?

Background- I have one friend, Mandy who is rather on the pushy and arrogant side. Sally, our other mutual friend got in a huge fight with Mandy over miscommunication and some stupid boy. Now Mandy, being the pushy person that she is, decided to go behind my back and get three of the other girls' I am rooming with on a trip to let her stay in our room (so now instead of 4 people to one bathroom its turned into 5). She didn't ask me at all, then texted me saying "I'm staying there whether you like it or not." Apparently she decided she was ditching Sally two months ago, but never asked me at all. The room is in my name. True, myself and the other three people paid an equal amount, but it still is rude and pisses me off. The other "roommates" i have claim that they didn't know what Mandy was doing, so that is why they failed to inform me of the situation (which is probably true since Mandy never makes up her mind about anything until the last minute). I think Mandy is just being a selfish princess beotch as usual. The pathetic part is, no one seems to think anything is wrong with letting Mandy stay (except me). So now it looks like I have to deal with her arrogant *** for five days straight. This really annoys me. I told Mandy I wanted to talk to her, but she hasn't gotten back to me at all in regards to it. I just dont want to deal with her crappy pushy attitude. She does this all the time. She thinks complaining and whining and being selfish will get her everything in life! Also its as if no one believes me when I say she is being rude and has an attitude. ... its like there are no principles or anything. It really ticks me off.