I am currently on a lease with 3 people but for months, my roommate has her girlfriend over at least 4 times a week (about 15 days a month). She also showers and cooks and stays in the apartment when my roommate isn't even around.

It wouldn't be so bad if it weren't for the fact that they come in at all hours, have loud sex and my roommate does not help out with the apartment at all.

We recently spoke to our roommate about this, mainly that we feel we are paying rent and utilities for a 4th roommate. She became angry especially since my boyfriend and I mentioned splitting the rent 4 ways and said she would keep the girlfriend around less.

That was two weeks ago and since then the girlfriend has spent 7 of 14 days at the apartment, even trying to hide in the apartment so that my boyfriend and I didn't notice she was around.

Can my boyfriend and I legally get my roommate to pay more for adding this new roommate?

Is it possible to get the landlord involved about this seeing as our roommate has added another person to our apartment?

Since you are all equally on the lease, you have no power to change anything. If you get the landlord involved, he may decide to evict all of you if having frequent overnight guests violates the lease.