Just to play devil's advocate here, isn't doing this kind of a strain on other people? In many cases it will cause everyone behind the group adding people to have to wait longer unless I am missing something?
If I was waiting in a party of 4 and then I was told it would be another 15 minutes because the party of two being seated brought two more friends in at the last minute, and now needed a 4 person table it would be kind of frustrating.
Just a thought.
Not if they give me the same table they had planned to in the first place, which is what I think will happen. Also, it might be a "strain" but I think Disney is more about sharing experiences with family and friends and would not have a problem with this. Even if you look at it from a monetary stand point, they're going to be selling more food to a couple more people, and even if you have to wait are you really going to cancel your reservation?