No, not unless there is a lease provision allowing him to do so. You're entitled to 30 days notice of a rental increase anyway. Check to see if your town has a rental ordinance; many do in CA and your LL may be in violation.
Maybe yes, maybe no. If you have a month to month tenancy, then 30 days notice will usually suffice for an increase in rent. (The amount of notice required is sometimes varied depending on your County) If they have given improper notice, then you can probably count on them simply correcting their mistake and/or simply terminating the tenancy in the event that you do not agree to pay the new amount. (sorry for the run-on sentence) If you have a lease for a specific term, then you should check to see what your lease says about visitors. If it is silent or if long term visitors are allowed, then you should not be required to pay the increased amount. Good Luck
The other lawyers are correct, expect that if your landlord wants to raise your rent 10% or more, he has to give you 60 days written notice of the increase. If he is raising the rent less than 10%, he has to give 30 days written notice. There is no provision in the law in California that triggers an automatic rent increase because someone comes to visit you.