You can always remove guests from your reservation without a penalty or cancellation of your reservation.
You can remove guests at any time from your Walt Disney World Dining Reservations simply by calling Disney Dining (407) 939-1947 or by changing the number of guests in “My Disney Experience”.
To “ADD” guests however, can be a little more difficult. You call Disney Dining at (407) 939-1947 and ask if it’s possible, but since it is based on availability, it is not guaranteed. The further out you attempt to add more guests, the better your chances. Always try to book your dining reservations for the largest number of guests possible to try (you can always delete people if needed later) and prevent missed seats.
Remember, one way to avoid these hassles is that Destinations With Character Travel Services can handle making all of your Fast Pass+ and Dining reservations for you!