Divide the grapes among four dessert dishes or wineglasses; set aside. In a medium bowl, mix gelatin with sugar. In a small saucepan, heat juice to boiling. Add to gelatin-sugar mixture and stir until gelatin is completely dissolved. Pour 1/2 cup gelatin liquid into each dessert dish or wineglass. Refrigerate 3 to 4 hours, or until mixture is slightly set. Serve cold.