My experience at many different WDW restaurants is that when you check in they ask "Is your entire party here?". If not, they don't even put you in the queue for a table and ask you to let them know when everyone is there. I'm sure it's because the restaurants are just too busy to have a table with only part of a party sitting there not ordering because they're waiting for others to arrive (finish riding a ride, etc.)
So my experience is that they will not seat you until the entire party arrives.