4 mint sprigs |
1 tablespoon superfine sugar, or to taste |
2/3 cup fresh grapefruit juice |
1 1/2 tablespoons fresh lemon or lime juice, or to taste |
3 ounces light rum (6 tablespoons), or to taste |
chilled club soda or seltzer water |
angostura bitters to taste |
garnish: grapefruit wedges and mint sprigs |