Your quality of life is only as good as people around you. Learn to avoid people who are cheap and more importantly learn to speak up. If you feel someone is not fair learn to tell them straight to their face. Best thing you can do is learn art of satire n believe me it works.
Like you said your roomies going to restroom when its time to pay. Next time its time to pay, say " restroom janey koi cha bhane bhana, pay garne bela bho."