In a small saucepan, combine the brown sugar and water. Heat gently over medium heat, stirring just until the sugar fully dissolves. Remove from heat and add the rosemary sprigs and grapefruit peel. Let steep for 6–8 minutes only, just until aromatic. Remove and discard both rosemary and peel. Let the syrup cool completely.
In a pitcher, combine the fresh grapefruit juice and 80–100 ml of the cooled rosemary grapefruit syrup. Stir gently and taste. The base should be bright, lightly sweet, and clearly bitter — not syrupy.
Add plenty of ice to the pitcher. Pour in the chilled club soda and stir once or twice only to integrate while preserving carbonation.
Pour into ice-filled glasses and garnish with a rosemary sprig and a slice of pink grapefruit.
