Add the red grapefruit juice, lime juice, agave syrup, half the pink grapefruit soda and 60ml (2fl oz/¼ cup) tequila to a Creami tub. Stir, cover and transfer to the freezer for 24 hours, or until frozen through.
When ready to serve, run half a lime around the edge of each glass and dust with Tajín.
Pour the remaining pink grapefruit soda into the tub