You should respect the boundaries of your roommates.
Mary is having a friend over to the apartment where she and her roommate share.
Mary wants to hang out with her friend.
Mary and her friend spend the night watching television in the living room.
Mary and her friend hang out in her friend's bedroom watching her television.
Mary's roommate enjoys the privacy of her room when she gets home.
Mary's roommate moves out because of the invasion of privacy and Mary has to pay rent herself.