Call Guest Services. They can actually do things like this if it is physically possible.
The similar story that I always tell is about one of our reservations at CRT.
When I made the reservation, my husband didn't want to be included. I spent the next 6 months asking him every now and then - but he always was adamant about not wanting to go. So....in the car driving down, he suddenly wants to go. Geez. I check the app - he can't be added. I check the computer - he can't be added. When we got there I went to Guest Services - they added him. I would call them.
I am assuming that you have tried modifying the number of people yourself.