We just call them "ushers" here but I guess you mean a specific event type usher but still the same definition.
Ushers escort guests to their seats before the ceremony (guiding female guests by the arm), roll out the aisle runner (if you have one), and act as general guest guides. Designating friends and family members as ushers is a great way to include more people in your wedding day fun without overdosing on groomsmen.
Ushers: Their Duties in Detail - The Knot