Make this rum-and-ginger drink ahead, if you like. Follow directions through Step 1; cover and chill up to 24 hours. Proceed with Step 2 when you're ready to serve.
Combine first 4 ingredients and, if desired, bitters in a pitcher; stir until sugar dissolves.
Pour mixture into 8 (10-oz.) rocks glasses filled with ice; top with club soda.
Adapted from The Lee Bros. Charleston Kitchen, copyright 2013 by Matt Lee and Ted Lee.