Tracking/Reporting Invitees vs Attendees (CiviEvent)
You could create custom Event participant Status called Invited. In the settings set the Class to Waiting and leave the Counted checkbox unchecked. When you mail out the invitations register all the people to the Event with the status of Invited. As you receive the confirmations go in and set each to attended. Then you can do Event reports based on who has attended or not.