I had my son and my friend join me mid-trip during December and yes, they did use Magical Express.
I broke my reservation into three reservations.
First few days I was there by myself, drove down with all the gear, reservation #1. (Wed night and Thur night)
Reservation #2, my son and friend arrived and I reserved Magical Express, and paid the extra adult charge. (Fri, Sat, Sunday), my son also flew home on Magical Express.
Reservation #3, my friend stayed on and rode home with me, (Monday - Saturday).
I had to break it up this way, or else I would have paid the extra adult charge for the entire stay.