In a large saucepan, bring 1 cup each water, sugar and fresh mint leaves to a boil; lower the heat and simmer for 15 minutes. Let cool, then strain, pressing to extract any liquid; refrigerate. In a pitcher, using a wooden spoon, muddle 3/4 cup mint leaves and 2 tablespoons sugar. Stir in 4 1/2 cups chilled club soda, 1 cup mint syrup and 3/4 cup fresh lemon juice (about 3 lemons). Strain into ice-filled glasses and top with a mint sprig.