When you are on the registration of a vehicle in California which is involved in an accident you can be liable for damages caused by the driver of that car. The law limits your liability to the lower limits of the required liability insurance, unless you are in some other way legally held liable. If you had some part in negligently entrusting the vehicle to a party that should not be behind the wheel you may have additional exposure to liability for negligent entrustment.
Its a good policy to limit your exposure by not being on the registration of vehicles you do not need to be on, or be on as a lien holder if you have only a financial interest. It is always a good practice to have adequate liability insurance when any motor vehicle is involved as the potential for high dollar liability looms.