Seat filler
A seat filler is a person who fills in an empty seat during an event. There are two types of seat fillers:

A person who subscribes to a seat-filling theatre club. Members of these clubs help fill in unsold seats for theatre, music, film, sporting events, dance performances and other live events. The producers of the event give complimentary tickets to the seat-filling organization, who pass them on to their members. The producers get a fuller audience and therefore a better experience for the paying patrons (and talent), while the seat-filler is able to see an event for a small service charge.