Run a lime wedge halfway around the rims of four margarita glasses and dip into salt. Set aside.
Combine 7 ounces grapefruit juice, one-half ounce lime juice, 2 1/2 ounces tequila and 1 ounce triple sec in a cocktail shaker. Shake over ice until chilled. Strain into two ice-filled, salt-rimmed margarita glasses.
Garnish with a twist or slice of grapefruit. Repeat to make two more cocktails.