Place mint, lime and agave gyrup (about 2 to 3 teaspoons, or to taste) in a sturdy glass. Use a muddler to crush the mint and lime to release the juice. Place ice in a beverage shaker and add mint mixture, rum and grapefruit juice. Shake well and seve over ice in a high-ball glass. Top off with club soda. Garnish with a lime wedge and sprig of mint.