Combine the grapefruit juice, lime juice, tequila, Cointreau, egg whites, and simple syrup in a blender. Blend until well-combined and frothy.

Spread some Tajín on a small plate. Cut a slit in the lime wedge and rub it along the rim of a glass. Roll the edge of the glass in the Tajín to coat the rim.

Carefully fill each glass with ice and pour in the margarita.