One who ushes.
In the theatre, there is a tradition of using volunteers to usher. If you want to see a show for free: Call the theater's box office for ushering information. They may have a regular pool of ushers or accept individuals. There's usually a dress code, and a set time to arrive early. In addition to taking tickets and seating patrons, you may be asked to help set up or serve at the concession stand.