Hi everybody. So I live in a townhouse in Washington, D.C. with four other housemates. One of them, who has consistently been a problem, let a friend of his move into his bedroom without informing anybody else in the house. He said that the other guy will be living there temporarily for two months until he can get back on his feet. He's classifying the new guy as a "guest", but the new guy has keys and everything.

I'm concerned that, by living in the house for so long, he would be considered a legal tenant. The roommate in question is very unreliable at best, and a pathological liar at worst, so I don't really trust that 1, this guy is a guest, and 2, this guy can be kicked out at any time. I tried looking up information about guests and tenancy in DC, but didn't find anything.

So I've got three questions:

1. Will the new person still be considered a "guest" after living in the house for two months, paying utilities, and having a housekey?

2. Do I have any legal recourse to stopping this guy from staying in my house (the landlord doesn't speak english well and doesn't communicate promptly, so he's not much help)?

3. If this guy damages/steals/whatever anything, is everybody in the house liable, is just he liable, is the guy who invited him in liable, or is the landlord liable?

Thanks for your help