The most popular term in insurance is "it depends". There is no way to make a blanket statement that will cover ALL insurance policies. To find the most proper answer for this question, you must look to your insurance policy. 

Most preferred insurance policies operate under the term “Permissive Use” This means those people, that are not directly excluded from your policy and have your implied permission to use your vehicle are covered on your insurance policy. 

Typically, if someone is a licensed driver in the home, or has regular access to the vehicles, it is best to list them on the policy. Check with your insurance agent or review your policy to find out the details of your coverage.