IN a small saucepan over medium low heat, add the water and sugar. Stir. Bring to a boil, stirring often. Once the mixture has boiled, remove from heat. Let cool. *
In a pitcher, combine 1 cup of the sugar syrup with 4 cups of grapefruit juice**, and 4 cups of club soda. Stir to combine. Place in the fridge until ready to serve. If serving immediately add ice cubes to chill the drink quickly.
Notes
*I let it cool for a few minutes, and then I place the sugar syrup in the fridge to chill it faster. **I used fresh grapefruit juice, and ran my grapefruit juice through a sieve to remove the pulp. If you like pulp, you don't need to do this step.