Chill 4 glasses (ones that hold at least 10 oz / 1¼ cups / 313 ml) in the fridge or freezer (see Chef’s Tips).
Optional: In a small, shallow dish, mix the crushed hibiscus flower and salt. Run a lime or grapefruit wedge around the rim of each glass. Dip the rims into the hibiscus rimmer.
Fill chilled glasses halfway with ice.
In each glass, slowly add the tequila (¼ cup / 63 ml), grapefruit juice (¼ cup / 63 ml), club soda (¼ cup / 63 ml), and lime juice (1 tablespoon / 15 ml). Top each drink with 1 tablespoon of the hibiscus syrup.
Garnish with grapefruit, lime, or hibiscus, if desired. Stir before serving.