It is no problem, just give them the flight numbers and dates when that extra person is joining you. As long as they are on the resort reservation, they will take care of everything.
Just as an example, in December, I am driving down on the 2nd, my son is arriving by plane on the 8th and departing on the 12th and will use Magical Express. I have a friend arriving on the 10th and staying through the 17th. She will use Magical Express under my reservation to arrive.
I'm leaving on the 16th and driving home. She is transferring to a different reservation for the night of the 16th and taking Magical Express to go home.