Moisten rim of 1 cocktail glass with grapefruit slice. If desired, sprinkle salt on a plate; dip rim in salt. Fill glass with ice.
In an empty cocktail shaker, combine tequila, Triple Sec and grapefruit juice. Fill with ice; cover and shake until frost forms on the outside of the shaker, 15-20 seconds. Strain into prepared glass and top with grenadine (it will sink to the bottom of the glass). Garnish with grapefruit slice.