If you like a salt rim on the glasses, rub the lime around the edge of the glass and then dip the rim of each glass lightly in a plate of kosher salt. Set aside to dry. 

Combine the grapefruit juice, lime juice, orange liqueur, and ice in a blender and puree until smooth. Pour into a large pitcher and stir in the tequila. If you’re not serving the margaritas in salted glasses, stir ¼ teaspoon of salt into the pitcher of margaritas. Serve ice cold.