Pour limeade into a large pitcher with a lid; stir in water, rum, grenadine syrup, lime juice, and bitters until well mixed. Cover pitcher and freeze punch until slushy, 8 hours to overnight.
Fill individual serving glasses with ice; Spoon about 3 tablespoons punch-slush mixture over ice. Top with club soda and stir gently.
To serve as a punch for a group – spoon or pour punch base into large punch bowl. Add ice and a 2 liter bottle of soda. Mix gently.
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