In a pitcher (at least 2-qt. capacity), combine 3 cups ruby grapefruit juice (fresh-squeezed or purchased), 2 cups tequila, and 1 cup triple sec or other orange-flavored liqueur. Chill until cold, at least 1 hour, or up to 1 day. Pour about 1/4 cup sugar on a rimmed plate. Cut a ruby grapefruit in half and rub rims of double old-fashioned glasses (8 oz.) with cut side of one half to moisten, then dip glass rims in sugar to coat. Fill glasses with ice cubes. Pour grapefruit margaritas over ice, taking care not to disturb sugared glass rims.