Red grapefruit juice and Cointreau combine for a margarita-style drink. As the name implies, a dash of salt is put in the cocktail or on the rim. Make the juice mixture up to one day ahead, and chill.
Combine juice, tequila, and liqueur; stir well. Coat rim of each of 12 glasses with 1/8 teaspoon salt. Fill each glass with ice; pour 1 cup juice mixture into each glass. Garnish each glass with a lime slice, if desired.