The invitations to the event were sent out by Dwight Drake, and seemed to include a lot of his friends. Beyond that, it was staged as though "Candidate" Colbert were making a pitch to S.C. Democrats to let him onto the ballot for the state’s presidential primary.
Dwight had hinted ahead of time that one Republican would be there for a special presentation, but it didn’t work out because the GOP token had a family emergency.
See the above video as a partial guide to who was there. At bottom, you’ll see a clip that shows Colbert making his "pitch" to the party honchos.