Frozen Grapefruit Margaritas

Slice 1 grapefruit into thin wedges for garnishing. Squeeze enough of the remaining 2 to 3 grapefruit to get 3/4 cup juice. Combine the juice, tequila, Cointreau, lime juice and ice cubes in a blender; puree until smooth. If desired, rub the rims of 6 cocktail glasses with 1 squeezed grapefruit half, just enough to wet them, and dip into salt to lightly coat. Divide the margarita mixture among the glasses and garnish each with a grapefruit wedge.