Use a citrus reamer to juice the grapefruits and lemons in a large or you can squeeze them by hand (that's how I did it!). Use a fine mesh strainer to remove any pulp or seeds.
Add the sparkling water to your juice and mix well. Transfer to serving glasses with ice and finish with rosemary sprigs.
Notes
For smaller servings, cut this recipe in half. Store in an airtight container in the refrigerator.