We just purchased a house in California and the previous tenants still want to live there a couple months until they find a new place. This is fine with us, however they refuse to give us a copy of the keys to our new home. Do we have the right demand a copy of the keys?
sounds like trouble waiting to happen but as long as your ok with it, I guess it's ok.
since they are tenants, you have now become their landlord. As such, you do have a right to keys to the premises. That doesn't mean you have a right of entry (haven't checked California law for that specific issue) but you might.
Unless there is a lease existing, I would simply tell them to allow you to make a set of keys (your expense of course) or give them notice to terminate the tenancy (either 30 or 60 days depending on how long they have lived there). Their choice.
edit: I would suggest reading up on California landlord tenant law so you follow the law in any interaction and when the tenancy terminates, you follow the law in regard to their deposit and any claim of damages you might have.