I have a 10 minute safety briefing in witch I explain what are the do and don't while a passenger. I had a green passenger for a 350 mile ride Sunday, she was the best passenger I ever had, I really had to check from time to time that she was there.
I don't have time right now, but I'll update my post with what I say in that briefing.
I wouldn't suggest telling the passenger to lean. All it really takes is for them to look over your inside shoulder on the side you're turning towards. They just need to stay with your position.
I guess if you were actually leaning off the bike, the passenger should mirror this motion like we've seen in some crazy pictures.
However, if you are mostly in plane with the bike as you lean it into a turn, the passenger should stay in that same plane. Leaning off axis either way will mess with your balance as a rider.