Have you talked to your room mate about this? I just think that before you do anything you should probably take her to the side and tell her that having 20 of her boyfriend's friends over isn't acceptable to you. Sounds like she is a bit inconsiderate. It's common courtesy to check with your room mate before having a party. So, at the very least she should have told you they were coming. But, certainly you can agree that because she is paying half of the rent, she should be entitled to an occasional party......but if you guys talk about it first then you can make plans to be elsewhere.
At any rate, if she doesn't see that it's a problem to disrespect you by not talking about it before having a large group over.....then you should likely be looking for another living situation.
just my little opinion..
Good luck.