Recently 10 foreign students moved into a 2 bedroom apt downstairs from mine. I have 2 roommates and 1 of them recently became friends with everyone in that apartment. Now i have half of them coming in and out my apartment at all hours. My roommate told 2 of them they could come in and use his computer when he's not here. Friday my other roommate and i were in the livingroom all day (which never happens cause its empty) and we encountered 4 of them coming in one at a time. None of them even said hi. We talked to our roommate and told him we were not cool with them coming over when he isnt here and that we would start locking the door. He wasn't happy about that. I recently got a kitten before they showed up and now they (or him) keep stealing her away for extended periods (sometimes overnight) by keeping her in his room upstairs even though shes my cat. I also mentioned this to him and that its not fair to keep her from freely getting to her food and litterbox. He got defensive and pissy about that too.
We are both on the lease. What can or should i do?