To a blender add ice, tequila, lime juice, grapefruit juice and simple syrup. Blend until frothy, about 30 seconds. Mix salt and the zest of one lime and add to a plate. Run a lime wedge around the edge of a tall glass. Rim the glass with the salt and lime zest mixture. Pour frothy lime and grapefruit mixture from the blender into prepared glasses. Top with beer and enjoy!