My two roommates and I have been living in this rental home in California for the past two years. One of the roommates is studying abroad and sublet his room to someone he knows without signing a sublet agreement with the subtenant or the landlord. The new roommate has refused to pay the rent he had agreed to, and he won't leave either. What are our options?
You have two things to worry about here: The validity of the sublease arrangement, and the new resident's refusal to pay the rent. While it must be galling to not receive rent contributions from the newcomer, how the first issue resolves itself might actually be more important to the very continuation of your tenancy.
You mention that the newcomer didn't sign an agreement with either the departing tenant or the landlord, but don't tell us whether there was an oral understanding among all three. If there was, it's valid and enforceable, for up to one year. In other words, the landlord gave his consent to this arrangement (a sublease between tenant and subtenant), which is crucial. The reason his okay is important is that your own lease, like most leases and rental agreements, probably requires the landlord's consent to any proposed sublet. Tenants' failure to obtain that consent is usually grounds for termination and, if necessary, eviction.
If your landlord gave his oral blessing to this temporary sublease; or if he was not involved but decides that he doesn't care if and when he finds out, you've dodged a bullet.
Assuming the subtenancy was approved (before or after the fact), the next issue is how to get the subtenant to pay his share. Realize that as far as the landlord is concerned, the rent must be paid by any one of the original three tenants, courtesy of the rule of “joint and several liability.” The landlord cannot, however, demand the rent from the subtenant, because he has no contract with him—he has merely given consent for one of the original tenants to act like a landlord and rent to the subtenant. So while you wait and argue with the sub, you must pay the full rent or face termination for nonpayment.
Meanwhile, you'll want to deal with the subtenant, but here's the problem: The sublease arrangement is between the departed roommate, now overseas, and the subtenant. If the missing subtenant were available, he could demand rent and then terminate and evict the subtenant, just like any landlord. But because he's not local, that will be impossible. And because you have no legal relationship with the subtenant, you can't take these steps yourself.
If the subtenant stubbornly won't move, consider contacting the departed tenant and demand that he pay his share, an obligation he kept even if he moved out for a while. It's up to him to get his subtenant to reimburse him. If he refuses, you'll have to come up with the rent between the two of you. Things will really get interesting when the original tenant returns—it's not unheard of to have the subtenant refuse to leave even then.
In the future, be sure that you thoroughly understand the subtenant arrangement, and that any temporary subtenancy is backed by a written agreement and the promise by the departed tenant to continue to pay his share, just as he did when he was in residence. If the subtenant fails to pay the departed tenant, that becomes the departed tenant's problem—but your tenancy will not be placed in jeopardy. Check out Nolo's guide for California tenants, California Tenants' Rights, for more information.