Hi, Kristen! If you can book a new reservation adding the additional guest, that would be the best thing to do. I understand, however, that it may not always be possible to book a new reservation, especially if you are planning to go to a popular restaurant.
It has been my experience that whenever they can, the cast members at the restaurants try to accommodate an additional guest. There have been times when I've found myself with one more person than I expected in my dining party, and I usually try to arrive a little bit earlier than my 15 minute window to see if there is any chance of adding the person to my party. I have found the cast members to be helpful and courteous, and even if I've had to wait a little bit longer, they have managed to accommodate my extra person.
I would go ahead and try to rebook what you can, but in the instances where you can't, just go to your reservation and let them know that you were incorrect in your original count and need to add someone to your reservation. Just give them a little bit of extra time in case they need to set an extra place or move some things around to add an extra chair. They really are very willing to work with you.
I hope you have a wonderful vacation!