Spread kosher salt on a plate. Rub the rim of a highball glass with a lime wedge; dip the rim of the glass into salt.
In a Boston shaker, combine Arrogante Reposado, Campari, agave nectar and freshly squeezed juices of lemon, lime and red grapefruit. Add a handful of ice and shake vigorously for 10 to 15 seconds.
Strain the drink into the prepared glass through a fine mesh strainer. Top the drink with Jarritos Grapefruit soda and stir gently. Add a small pinch of kosher salt and garnish the cocktail with several small pieces of red grapefruit.