Elderflower is the hidden treasure in this rum-spiked take on the traditional Salty Dog.
Squeeze juice from grapefruit quarters into a measuring cup to equal 5 to 6 cups juice; pour through a wire-mesh strainer, discarding pulp. Combine rum, liqueur, and juice in a tall pitcher. Set aside.
Squeeze remaining juice from grapefruit onto a rimmed plate. Combine salt and sugar on another plate. Dip rims of lowball glasses into juice on plate; dip rims in salt mixture to coat.
Add ice to pitcher, and stir contents vigorously until chilled. Strain into prepared glasses; garnish, if desired. Serve immediately.