Prejudgment Claim of Right to Possession - a form that a landlord in an unlawful detainer (eviction) lawsuit can have served along with the summons and complaint on all persons living in the rental unit who might claim to be tenants, but whose names the landlord does not know. Occupants who are not named in the unlawful detainer complaint, but who claim a right to possess the rental unit, can fill out and file this form to become parties to the unlawful detainer action.
You'll find the CA Dept of Consumer Affairs Guide helpful - (first link below, scroll down to the heading Landlord-Tenant and click on the downloadable PDF titled "California Tenants - A Guide to Residential Tenants' and Landlords' Rights and Responsibilities")
Over at the CA Dept of Real Estate website, Chapter 9 is Landlord-Tenant as well - see link 2 below.
Find eviction-related legal aid near you and self-help resources by following the third link below.
Same result (but shorter link) if you just go here and search under Landlord-Tenant and enter your city or zip code - see link 4 below