To make the simple syrup, combine water, sugar, and rosemary in a saucepan. Bring to a boil, turn off heat, and let steep for 10 minutes. Remove rosemary. Store in the refrigerator for up to a week.
To assemble the drinks. fill 4 glasses with crushed ice. Add ½ cup grapefruit juice to each one. Top with ½ cup Voveti Prosecco. Garnish with fresh rosemary, if desired.