Oh...and a piece of advice....don't call reservations and attempt to change it....the CM there was so befuddled about how to do it, talking about there being no openings and she'd have to cancel my original reservation and couldn't guarantee we'd get it back with the higher number (from 3 to 4), etc....I finally said, never mind, I'll just tell her she can't come and hung up, scared to death we'd lose it all! Then we decided to just go to the podium and tell them we had a 4th person. We did this all week long and never were we turned away. At a couple of places I felt like we were waiting longer than others who had come in after us, but then again I don't know that they didn't have earlier ADRs in the first place. Never had to wait what I considered unusually long though. The one place that they seemed a bit confused about how to do it (but assured me it was fine) was CRT for dinner....but that was because it was prepaid. They did the weirdest thing, instead of just charging my card for one dinner....they REFUNDED me the 3 dinners we'd prepaid and then charged me for 4 dinners. Whatever, it came out to the total I expected so I didn't question it, but it stills seems an odd way to do things, lol.
I think the problem comes when your addition would change the size of table....going from 2 to 3 for instance, or 4 to 5....then the table they alloted for you would no longer work. But, I'm guessing that they'd still find a way to make it work, I think in that case though I'd be prepared for extra waiting, or the real possibility of being turned away, especially at the busier places.
I'm trying to imagine what a table for 3 would look like anyway, lol....a triangle? I saw many tables that were probably meant for 3...they were up against a wall with 3 chairs....but could probably have been moved and another chair found to make it 4 with only a little bit of work.