Add a slice of fresh grapefruit to the bottom of a 60-ounce pitcher. Fill a quarter of the pitcher with ice. Add another slice of grapefruit and another layer of ice. Repeat this until the pitcher is full, ending with a slice of grapefruit on top. Pour in the grapefruit juice, elderflower liqueur and bourbon and stir. Add any remaining grapefruit slices, reserving 2 for garnish.
Slice the 2 remaining grapefruit slices into half-moons and place on the rims of tall pint glasses or wine glasses. Fill the glasses with ice, then fill them halfway with club soda. Top with batched Grass Roots mix and stir. Straws optional!