I was thinking that there might be a publicly known guest (on which they did not appear) list and a private guest list (on which they did appear). As some aluded to on the thread, this could be a setup to try to make the secret service look bad. What better way to start a "civilian security force better equipped than the military" than something like this?
Why did not they eat, then, and why did they not sit down (i.e., reported no place for them). That seems rather awkward. One wonders if there were name cards for each guest on the table with their tableware. In that case, no invitations— then no formal place cards, no formal place cards—then no seats, no seats—then no seats at the table, no seats at the table-—then no sitting down. With all the bevy of other sycophants from White House staff, interns, volunteers, photographers, hired help, security, they could easily just kind of hang along the side and cling to that dinner; everyone else would be so involved in the food and conversation at their table and the speeches to even notice them perhaps. Who knows?