Most leases have a clause that makes all tenants “joint and severally” liable. That means that:
Everyone who signs the lease can be held responsible for the entire amount of rent, not just his/her share. If one of your roommates fails to pay his/her rent, the landlord can make YOU pay your roommate’s share of the rent.
Similarly, everyone who signs the lease can be held responsible for damage to the property, even if they had no part in causing the damage.
Finally, under a joint lease, a violation of the lease by one tenant may be considered a violation by all tenants. For example, if one of your roommates violates the lease by having an unauthorized pet, the landlord could evict everyone on the lease.
Because most leases are joint leases, it is important to choose your roommates wisely. Choose roommates you can trust to be responsible. It is also a good idea to come up with a “Roommate Agreement” that outlines each roommate’s expectations of the others including, cleanliness, noise, visitors & overnight guests, rent, utilities, etc. You can find a sample agreement here: http://www.nolo.com/legal-encyclopedia/renting-house-apartment-with-roommates-29865-2.html