If you let someone drive your car occasionally, you probably do not need to add them to your insurance. As long as you give the driver permission to borrow your car, they will likely be covered in case of an accident. However, if you expect someone to drive your car regularly, you may need to add them to your insurance policy. Contact your insurance for more information.
One exception is when the temporary driver lives at your house. If a family member who lives at home will regularly borrow your car, your insurance company likely requires their name on your policy to ensure coverage.