Not worth it to be honest! If plod stop them driving your car, they'll be done for no valid insurance unless they can prove in writing there and then on the spot. That means car seized, £200 fine (if they still have uk licences) and 6 points!!!
Or worst case scenario, heaven forbid, they have a bump. Then your in a whole new world of pain!
Just add them to your policy as named drivers, I'm sure it won't cost the earth.