Combine fresh grapefruit juice, Cointreau, tequila and sugar. Stir to combine. Cover and refrigerate until cold, at least 1 hour.

Pour enough salt into a saucer to cover the bottom. Rub a lime half over the rim of each champagne glass and upend into the salt to crust it lightly. Pour about 3 ounces of the tequila mixture into each glass; fill the rest of the way with Champagne.