Cast iron works on the grill just like it does on your kitchenís stovetop:
Place it directly on your grillís grates and let it get ripping hot. Pour a tablespoon or two of a neutral oil into the skillet and wait until it starts smoking. Then place your patties in the skilletómake sure things donít get too crowdedóand step away from the grill and let your burgers cook, covering the grill (with the top vent open) for a minute or two during the process to let the patties pick up some smokiness. Cook till done salt & pepper to taste.