Hi, I rent a house in Maryland with 3 other people. We have been at the residence for 9 months now. My roommate's girlfriend has lived with us rent free for the whole 9 months, she doesn't even help with utilities. I am getting sick of this and I don't know what I can legally do. She has spent every night at our apartment and I want her to pay rent or to leave.

Have you talked to the room mate about this? What does he say?

Yes, they don't like it either but they don't want to cause any problems.

Two of you gang up on the guy with the girlfriend and tell him that she pays almost a 5th (because a bedroom is only a small part of the house) or she has to leave and you will notify the landlord that the house has an undesirable unwanted freeloader there (which will get all of you evicted). You could even talk to the landlord about sending you all a letter demanding more but you really just want help getting her out.
People renting together should have written rules about this sort of thing.

Lets get the facts straight. There are 4 people on the lease and the girlfriend is a 5th person? Is this a 4 bedroom house? How are utilities handled are they part of the rent or divided 4 ways or what?

How about groceries, does she eat only from her boyfriend's pantry?

The question was not whether you have talked to the other roommates, but to the roommate who is the boyfriend?