How many is the table booked for?
It's fairly easy to go from 3 people to 4 or 5 to 6 as most of the tables seat 2, 4, 6, 8. There are some tables that do seat odd numbers as they are up against a post or wall.
If you aren't going at a very busy time, then you probably won't have any trouble if you check in at the podium and just give them the new # of people.