For Magical Express, if you don't get them added prior to going, they can just go to the desk at MCO, and they will be able to get on. I believe they can then just give the bag tags from the airline to the DME people and they will collect the luggage or they can just bring it with them.
So, if it is last minute, as long as they are on the reservation day of travel, riding DME won't be a problem!