Place a saucepan over medium heat and add the water and sugar; mix until the sugar dissolves. Add the sprigs of sage and bring the mixture to a simmer, cooking for 5 minutes. Take the pan off the heat and allow the syrup to steep while you make the rest of the drink, about 10 minutes. After it's done steeping, remove the sprigs of sage and discard.
Before you juice the grapefruits, slice off four grapefruit rings for garnish. Juice the grapefruit juice into a measuring cup. If you happen to despise pulp (that would be me!), feel free to strain the grapefruit juice.
To make the cocktail, add about 3 to 4 liquid ounces of grapefruit juice and two tablespoons of simple syrup to a glass. Add two dashes of bitters, 2 liquid ounces bourbon, a splash of sparkling water and ice. Stir until combined, and garnish with the grapefruit peel. Repeat the process until all the other cocktails are assembled.