I let only two people ride my horse - his previous trainers who are now also my friends, wonderful persons and wise horsemen. I also feel safe with the fact that my horse likes them. However, due to the fact, that my horse is a "one person type only" guy, I don't let others ride him, especially, if they are just friends without experience who'd like to have a little ride. He's a bit too dominant to give him the chance to get all grumpy around novices - on a good day he'll just refuse do anything, but if he's really irritated and feels in charge, he can also bolt. He used to be a lesson horse for a short while before I bought him and I think that's where he learned to hate frequent change of inconsistent riders.
I have come a long way, to surrender my shadow to the shadow of my horse.