Place sugar in a shallow bowl. Moisten the rims of 4 highball or margarita glasses and dip into the sugar to coat. Add ice to each glass. Fill a pitcher with ice. To pitcher, add rum, simple syrup, and lime juice and stir until chilled. Strain into prepared glasses and top each with a splash of club soda. Stir to combine, then garnish each drink with a lime wheel and mint sprig.