I have added guests but it has been a very long time - 2012 I had originally made reservations for Minnie & Friends, and for the Princess Breakfast at Ariel's Grotto for just my family of (then) 5. Additional extended family was able to join, and I was able to call and add an additional 5 for each event - I did have to call, and I was happy to adjust our time (heck, even our day if it had come to it), but they were able to adjust our reservation.
I have absolutely no idea if this is remotely the same experience anymore, but you could try grabbing the time you want and then calling to add. Worst case you have to cancel it and keep looking!