Generally, you can add someone to your policy who has a driver's license, regularly uses your vehicle and lives at your address. Most insurance providers require policyholders to add household members when you first sign up for insurance. Sometimes, you can add drivers who don't live with you, but this varies based on your insurer and location.
Adding someone to your car insurance will affect your premiums, but the extent of the increase will depend on factors such as the other driver's age and accident history.