Most of this content is gleaned from Geico's website:
There is no need to notify the insurance company about this situation, however there are some rules and liabilities to be aware of before you lend the car to someone. In case of an accident, your insurance will be used meaning YOUR deductible and, if damages exceed your coverage, you could be liable for additional damages.
According to Geico, a large US-based insurer handling 10.8% of the market, "you should ask a few more questions before letting another driver head into the sunset in your vehicle—or before borrowing someone else’s ride."
If you’re loaning out your car, ask:
Is the borrower licensed to drive? If they’re visiting from out of state, you don’t need to worry. If they have a driver’s license from another country, check your state’s requirements—the driver may need to apply for an International Driving Permit before he or she arrives in the U.S.
Does the borrower have a good driving record? If they have a history of fender benders and you keep your vehicle in pristine condition, it’s OK to not hand over the keys.
Is my insurance up to date? Every state requires a minimum level of auto insurance coverage, and some stipulate that in case of an accident, the car owner’s auto insurance—not the driver’s—provides primary coverage. (So if the driver rear-ends someone or bumps another car in a parking lot and your insurance has lapsed, you could be liable for damages.) Depending on your state’s regulations, you may also want to verify whether the driver has his or her own car insurance.
What do they plan to use the car for? If it’s any kind of commercial activity, like driving for a ridesharing program, you’ll need to check your auto insurance policy. Again, in many states, the car owner’s auto insurance provides primary coverage. If the car is being driven for commercial purposes and you only have a personal policy, you may not have sufficient coverage.
Will the borrower use the car regularly? Your newly licensed teenager may phrase it as ‘borrowing’ the car, but if they’re using it to go to sports practice or the mall every weekend (and live in your household), you should add them to your insurance policy as a regular user.