When two or more people sign the same rental agreement or lease—or enter into the same oral rental agreement—they are cotenants and share the same legal rights and responsibilities. This legal principle, known as “joint and several liability,” has enormous implications for cotenants.
Keep in mind that here we are talking about housemates who are cotenants, not tenant and subtenant. This is an important distinction. A tenant and her subtenant (for example, someone who has sublet an apartment from you and has not signed the lease or rental agreement) do not share the same rights and responsibilities with respect to each other or to the landlord. See Legal Issues With Roommates for the legal definitions.