lace the 1/2 cup mint leaves in a pitcher. Use the back of a large spoon to lightly bruise the leaves. Stir in grape juice and pineapple juice. Cover and chill for 4 to 24 hours.
Strain mint from grape juice mixture and discard. Stir club soda into juice mixture. Pour into ice-filled glasses and, if desired, garnish with mint sprigs. Makes 6 (5 ounces each) servings.