For the salt: Crush the salt and grapefruit zest together in a mortar and pestle. Rim glasses with the grapefruit salt to serve the cocktails in.
For the cocktail: In a glass pitcher, muddle the raspberries, jalapeno slices, agave syrup and lime juice together. Add the grapefruit juice and tequila. Take a second pitcher of the same size and fill it three-fourths of the way with ice. Pour the drink into the pitcher with ice, then back into the first pitcher, repeating back and forth to mix together. Pour into the rimmed glasses, top off with club soda, garnish with grapefruit wedges if desired and serve.