There are some rules about friends borrowing your vehicle, specifically related custody for extended periods etc.
The question of whether someone else is covered under your policy, is primarily a matter of consent. If that person has consent to drive your vehicle from you, then cover should extend to them as well (though specifically for your protection). Realising however that this also comes with some obligation on your part to make sure that the person is actually licensed to drive your vehicle, and that the vehicle will be used in a fashion you have insured it for. Giving consent to an unlicensed driver, could cause some issues though.
If the friend has a vehicle insurance policy of their own, their policy will usually cover them even if they are driving your vehicle.
As was rightly pointed out situations are often unique, and coverages are also often fairly unique, so if you have any concern whatsoever about your own circumstances, call your licensed insurance broker, or insurance company. Explain your situation and get proper advice from them on your situation. This way you can be certain that it is what you need it to be.
Good luck
