Roommates who are not listed on a lease or rental agreement are considered to be subtenants (or "subletters"). Though subtenants are generally free of many of the restrictions placed on tenants with long-term leases, they also don't always have the protections against tenancy termination and eviction that tenants with written rental agreements and leases do.
A subtenant is someone who rents space from a tenant who has signed a rental agreement with the property owner. The subtenant is responsible for paying rent to the tenant, who then pays the landlord. The subtenant usually has no direct dealings with a landlord, though the landlord should be made aware of the presence of the subtenant.
In a subletting relationship, the master tenant is the tenant whose name is actually on the lease. The master tenant may sublet her rental unit entirely to a subtenant, or may only sublet a portion of her home and live in a roommate situation with the subtenant. The subtenant pays rent directly to the master tenant, and the master tenant can terminate the roommate's stay in the unit by following proper legal procedures.
In some areas of the United States, subletters are entitled to know how much rent is actually being charged for the rental unit and can only be required to pay a fair share of the rent and expenses.
Roommates who are not on the lease should ask to see a copy of the original lease. If the lease contains a prohibition against subletting, the roommate may not be protected under a state or city's tenant rights act. This can make the roommate vulnerable to a speedy eviction.
If a subtenant is not on the lease, state law may give the tenant who is on the lease or rental agreement the ability to evict the subtenant. The original tenant will still need to go through the eviction procedure mandated under both state and local law. In some places, the subtenant can only be evicted for the same grounds needed to evict a standard tenant. In other places, a subtenant can be evicted for almost any reason.