Cut each grapefruit in half horizontally.
With a sharp knife, cut around each section to loosen fruit.
Place grapefruit halves, cut side up, in a 15-in.x 10-in.x 1-in.baking pan.
Combine the brown sugar and 2 tablespoons sugar; sprinkle over grapefruit.
Drizzle with butter.
Broil 4 in.from the heat until sugar is bubbly.
For garnish, rinse the grape clusters and dip in the remaining sugar.
Place on the grapefruit; serve warm.