Fill a cocktail shaker with ice, then add fresh grapefruit juice, tequila, fresh lime juice, Cointreau, and agave. Secure lid, then shake for 10-12 seconds to aerate the drink.
Run a lime or grapefruit wedge along the rim of the glass, then dip upside down into a shallow bowl with equal parts Kosher salt and sugar stirred together.
Place ice in glass (optional). Strain into glass, then garnish with grapefruit and/or lime wedges.