They could turn away one person in your group, but it will be up to you to decide who that person will be. The castmembers at the podium will do their best to accommodate an extra person, but there is no guarantee it will be possible. We were at a Disney World restaurant that was running behind schedule and it was turning away walk ups, latecomers and anyone not on the reservation in an attempt to get back on schedule. Walking up with an extra person would be a last resort because no matter how many posters say that it wasn't a problem for them, there's no guarantee that you'll be as fortunate. It will depend on the restaurant, how busy it is at the time of your reservation and how busy it will be in Disney World when you are there.
On another post you mentioned going in December so you have plenty of time to keep checking to see if you can find a new reservation for your entire group. December can be very busy depending on when you're going so I would keep checking online frequently because cancellations occur at any time. You can also subscribe to the December cancellation thread at the top of this forum. Touring Plans has a reservation finder that many have used.
You could also look for a reservation for one at the same time as your current reservation. When you check in ask to be seated together, but again, there's no guarantee that you won't be seated at two separate tables since you have two reservations. The tables might be near each other or they could be on opposite sides of the room.
