Roommates: Living with Someone Without Being on Lease

My question involves a roommate in the State of: NY

My girlfriend rents a house with four other people in the town she goes to college. I have been staying here with her for about three months now, and the roommates know that and have been totally cool with it. Today, one of the roommates approached us and said that he doesn't mind me being here, just that he doesn't want to get in trouble if the landlord finds out that I am staying here. However, I have been here for three months and the landlord hasn't said anything about it. My girlfriend and I found the lease agreement and read through it a few times - nowhere in it does it mention guests or how long anyone is allowed to stay here, etc. So, my question is, since it does not explicitly state that I can't be here and the roommates don't care, if we don't say anything to the landlord (considering I have been here 3 months, he obviously doesn't care), is it okay that I am here? If I explain this to the roommate and he's like, "oh, okay" then that's fine, right? Even if someone did get in trouble for me being here, it would only be my girlfriend, right? And technically, it doesn't state anything about not being able to have guests in the lease, so she's not breaking any rules and whatnot.