Pour the tequila, lime juice, and agave syrup into a 2-quart pitcher and stir for about 15 seconds.
Cover and refrigerate the pitcher for at least 30 minutes and up to 24 hours.
Rub a lime wedge around the rim of each glass, then dip in coarse salt.
Fill each salted glass with ice.
Pour the tequila mixture over the ice, filling each glass about half full.
Top each glass with chilled grapefruit soda and stir once gently.
Garnish with a grapefruit wheel and serve immediately.
Good to Know
For the zero proof pitcher, replace the tequila with an equal amount of white grape juice and add an extra squeeze of lime. The grapefruit soda stays identical.