The unknown occupants have 10 days from the date they are served to file a Prejudgment Claim of Right to Possession form with the court and pay the required filing fee, and 5 days thereafter to file a response to the summons and complaint. Any unnamed occupant who does not file a Prejudgment Claim of Right to Possession form can be evicted.
Once 15 days have passed, you need to immediately file a proof of service of summons showing that you served all unknown occupants with the summons, complaint and prejudgment claim of right of possession along with a Request for Entry of Default against "all unknown occupants". (Code of Civil Procedure section 415.46).
If you don't do this, then you may run into the so-called Arrietta claim. Any tenant, not named in the summons and complaint, can assert this claim as late as when the Sheriff attempts to serve the Notice to Vacate on the named tenants. The unnamed occupant merely has to hand the Sheriff with the Claim of Right of Possession and the process stops until this person's claim can be adjudicated in court. The court must set a hearing within 5 days after receipt of the claim if no rent is posted by the claimant. If the claimant posts 15 days worth of rent, then the court can set the Areietta hearing from one to two weeks later.
The information presented here is general in nature and is not intended, nor should be construed, as legal advice. This posting does not create any attorney-client relationship with the author (who is only admitted to practice law in the State of California). For specific advice about your particular situation, consult your own attorney.