As a player, you should always be aware of who you play against. If you know that a match never occurred because your opponent wasn't there, or he/she was using someone else's name/DCI number, be sure to report it. If an event is falsely recorded with fake or "ghosted" players the event will be deemed invalid, which may negatively impact your ranking or event invitations.
If you are an event organizer and find yourself without enough players to run a sanctioned event, report your event as casual. Do not add players to the event that did not participate in order to "round out" the numbers. Casual events still count towards your Wizards Play Network level advancement.