Partygate (crash)

So, last Tuesday, as widely reported, a Washington couple decided to gatecrash Obama's reception for the visiting Indian premier Manmohan Singh. After arriving and posing for photos, including with Obama, they then went to have cocktails in the East Wing. It's known at no point were they thrown out, but how they could have had a seat at dinner is bizarre. These things are planed in advance, both for food and wine, but also for who sit next to who. It's obviously not just possible (is it?) to pull up a couple of chairs and share with your neighbour. A later article suggests the secret service is now considering criminal charges. Quite what for it's not clear. Is gatecrashing where you just walk up and say "let me in" illegal?