Hi there, Dorthy!
I'm so happy to hear that you've been working on your
Advanced Dining Reservations for your fall vacation to the Walt Disney World Resort! And how fun that your daughter and granddaughter might end up joining you as well!
In order to add people to your existing dining reservation, you'll need to call
Disney Dining at 407.WDW.DINE [939.3463] and they'll check for availability with your new party size. What they'll do is make the new reservation for you and cancel the old one right at the same time. It's been my experience that once a reservation is confirmed, they cannot add Guests to it.
What you can also do is make a separate reservation for just the two of them at about the same time as yours. When you check in at the podium, you can as to be seated together and the Cast Members will do the best they can to make that happen for you.
Either way, I'm sure you'll have a fabulous time. I hope this helps. Please let us know if we can help with any other questions you have.
Wishing you and your family a Pixie Dusted vacation!
~Jackie