With car insurance in Spain it is the vehicle that is insured. So if you have a family member who meets the insurers criteria there is no need to add them to the policy. For example, the insurance company may stipulate that a driver must be over 25 years of age and held a full licence for a minimum of two years. If your visitor meets that criteria then they can enjoy the same insurance benefits as you and there is no need to advise the insurance company.