I’ve been in this situation many times, Martha. It’s extra fun when a family member or friend decides to join us for a meal, but it’s not always easy to make that change to the
dining reservation.
There really isn’t a way to “add a Guest” to an
existing dining reservation. Rather, you’d need to check for a new reservation with the updated number of Guests in the dining party. If it's available, book it. As part of the process, the system will prompt you to cancel the previous reservation. You may find that a reservation for the new party size is
not available. If that’s the case, don’t lose hope just yet and don’t cancel the reservation unless you are willing to lose out on that dining experience.
Here’s what I recommend: if you’ll be at Walt Disney World for a day or two before the reservation, you might try stopping in at the podium of the restaurant and asking the Cast Member if he or she can accommodate one extra Guest. That may be a possibility and it’s the easiest fix. If not, you can also make one last attempt at the time of the dining reservation by showing up with the extra party member. If you choose this route, it’s important to prepare that extra person for the possibility of being turned away. Of course, the Cast Members will do everything they can to grant your request, but it’s not a guarantee.
I’m sending pixie-dusted wishes your way, Martha, for success with your dining reservation change!