The definition of an occasional driver can vary by state and from one insurance company to the next.
For example according to State Farm's definition, an occasional driver is someone unmarried and under 21 who drives the car less than 25 percent of the time, or puts on less than 25 percent of the mileage. A principal driver uses the vehicle 50 percent or more of the time. The insurance premium for a principal driver will normally be considerably higher than an occasional driver.
Some insurers suggest as a rule of thumb that an occasional driver is not found to be driving the vehicle in question no more than once a week.
The following is the definition from our website of both occasional driver and principal driver. An occasional driver is the person who is not the primary or principal driver of the vehicle and the definition for principal driver is the person who drives the car most often. These are the general definitions used in the insurance industry.
The requirements for someone to be termed an "occasional, casual or intermittent" driver differ by insurance company and possibly by state so it is best to check with your insurance agent for your specific company's requirements.
That being said generally insurance companies allow coverage for the occasional driver provided that driver does not live in your household and the vehicle is parked at your dwelling at the end of the day.
If you want to add a driver as an occasional driver to your policy you will need to discuss their definition of this term and how often they say a person can drive a car without being considered more than an occasional driver.