Pour Tajín seasoning into a wide shallow bowl or onto a small plate. Run a lime wedge around the rim of each serving glass to wet the surface. Dip the rim of each glass into the Tajín seasoning. Set prepared glasses to the side.
Add tequila, lime juice, orange juice, and agave syrup to a cocktail shaker with ice. Shake vigorously until cold.
Add ice to each prepared glass. Strain margarita into each glass.
Top with a float of Grand Marnier and a lime wedge for garnish.
