Spread 2 tablespoons of sugar on a flat plate. Halve the grapefruits and cut off a slice. Moisten the rim of 6 cocktail glasses with it, then turn the edges of the glasses in the sugar to create a sugar rim
Squeeze grapefruits and limes. Pour the juice into a blender, add 4 tablespoons of sugar and ice and mix until the ice cubes are finely pureed. Pour into glasses and add 1 teaspoon of grenadine syrup to each glass. Decorate glasses with slices of lime if desired.