When you arrive, surrounding the walls of the board gaming ball room is a thing we call "Big Board" It is a listing/signup sheet for EVERY event in the convention. Anyone who has pre-registered for your event will be printed on the sheet, with their badge number. During the convention, people may write their name and badge numbers in for any even that has slots to fill.
This is all explained in the "GM Rules" sheet which you will receive in your packet when you first get your badge at registration. All expected procedures, protocols, and such are on that sheet. If you don't understand anything on that sheet, again find a Senior Staffer, and they'll explain it out to you.