Pour a small amount of agave syrup on a shallow plate, and salt on another plate. Press the rim of the glasses into the syrup, then into the salt. Fill the glasses with ice and set aside.
Fill a cocktail shaker with ice. Pour the fresh grapefruit juice, tequila, Triple Sec, and agave into the shaker.
Cover and shake vigorously for 30 seconds. Then pour into the glasses. Garnish with fresh grapefruit slices and enjoy!