I remember learning during my ethics briefing (OMB, not congressional) that industry events frequented by feds and congressional staffers will oftentimes say on the invitation how much the catering will cost per person and provide a way for attendees to reimburse the event organizers. Most events will just serve coffee and muffins to avoid this (since it's generally considered kosher to take a small pastry and a beverage w/o needing to file an ethics disclosure or pay back the organizer).