How to Make It
Combine first 3 ingredients in a saucepan over medium-high heat. Bring to a boil; reduce heat to low, and simmer 15 minutes. Remove pan from heat; cool completely. Strain and discard solids.
To prepare each cocktail, combine 3 tablespoons celery syrup, 3/4 cup juice, and 1 teaspoon bitters in a cocktail shaker. Shake well. Rim 5 glasses with kosher salt, and fill with ice. Divide the celery mixture evenly among glasses.