The Post-Mortem meeting, also sometimes known as Hiss-and-Purr, is a feedback session where the convention gets to discuss what went right and what went wrong. These are typically held at the end of the convention, as a sort of last panel of the convention. If so, then audience participation will almost certainly dominate. If it is held at some later date, then it is more likely to be a mix of ConCom, department chiefs, volunteers, and others that want to attend.
Remember: Don't limit feedback from the membership! It is permissible to have more than one Post-Mortem session, and they can focus on the convention as a whole, the events of that day, or on specific aspects of the convention, such as Masquerade, Dealers Room, Art Show, etc. Getting feedback immediately after the event (or as soon as possible) is best, while the details are fresh in the mind.