Legal Question in Real Estate Law in California

I assume you're the landlord. Ask them to leave if they're not authorized to be there. Then, if they don't leave, start the eviction process, or unlawful detainer, against them.

Eviction. If you don't know how, hire an eviction service or attorney.

This state of facts may reflect an assignment of the lease; it may reflect a sublease, or it may simply reflect thhe presence of a gues and the absence of the tenant. Does the lease say anything about assignment, subletting, guests or identity of occupants? Who is paying the rent? I would be more cautious about viewing this as a situation that calls for, or even permits, an eviction.