Now is a good time to teach them, explain to them how you want them to be on the back of the bike, to hold on tight, but not too tight as their arms will tire fast. Lean when they need to lean, make sure you have some sort of way for them letting you know that something is wrong, or they need to move a little, maybe like a tap on your left thigh or shoulder, No sudden movements or waving to friends as they pass by. Start out slowly, GEAR IS A MUST!!! After a few months it will feel like they are part of the bike....
One thing to think about, if they do end up doing something wrong or that you dont like, you need to pull over then and there and explain to them, not in a yelling or loud tone of voice, but calmly explain to them what the have done and the way they need to correct it so it doesnt happen again.
Dont make the mistake I did, had my wife on the back of the bike, had talked with her time and time again about leaning with me and not against me, we where out on a ride, going through an hairpin switch back U-turn corner she got an uneasy feeling and went from leaning with me to leaning against me, upsetting the motion of the bike in the middle of the turn, I over compensated and almost dropped the bike. I ended up scraping the peg on the ground, put my left leg out to keep us from falling over nearly throwing us both off the bike, and hurting the crap out of my hip.
Read this forum topic about ridding with your girl on the bike look for the post from "AzizaVFR" very informative. Page #3 Post #23, the whole thing has some good comments, but by far his post is the best.
http://www.cbr250.net/forum/honda-c...there-such-thing-experienced-passenger-3.html