No, the employer may not force them to work for free: all work done for an employer (i.e. all work required by the employer) must be paid. Furthermore, all employees must earn at least minimum wage, so if there will be no tipping, servers and other tipped positions may have to be paid more than normal; and if they work more than 40 hours in the week including at this event, they must receive overtime. The staff *may* choose to volunteer their time--but they can't be required to do so.