For Concord Grape Shrub:
Cover Concord grapes with apple cider vinegar. Store in a cool, dark area for 1 week to 1 month, but allow to aerate. Check once a week for appropriate tartness. After 2 to 4 weeks, remove and strain into a medium sauce pan. Add sugar and over low to medium heat, bring to just under a boil to dissolve the sugar. Strain and cool. Bottle and keep in the refrigerator.
To Assemble and Serve:
In a highball or Collins glass add the Concord Grape Shrub and top with club soda. Stir and fill with ice. Garnish with a grapefruit twist.