Yes, in general you can add someone to your policy as a partial or occasional driver. There is no limit to the number of drivers you can insure. It is important to inform the insurance company of anyone that may operate your vehicle even if on an infrequent basis. You should contact your insurance agent or insurance company to determine what their procedure is for adding a driver.
It depends on the insurance company and their guidelines as well as their rating systems if someone can be listed as an occasional driver on your vehicle.
Not all insurance carriers rate using occasional driver or primary driver. An occasional driver is typically rated at a lower rate than a primary driver by insurance providers that do have these different designations. Also, it may be possible to have a person listed as the primary on one vehicle, but not on all household vehicles.
Regardless of cost, you will need to tell the use as defined by your specific insurance carrier of those that operate your vehicle.
You can get a safe auto insurance quote here to see how different drivers of different ages with different driving records may affect your premiums.