1. Pour 2 cups boiling water over tea bags; cover and let steep 5 minutes. Remove and discard tea bags, squeezing gently.
2. Stir together remaining 7 cups boiling water and 2 cups sugar in a Dutch oven, stirring until sugar is dissolved. Stir in tea mixture, orange juice concentrate, and next 3 ingredients. Let cool to room temperature (about 30 minutes). Divide mixture between 2 (1-gal.) zip-top plastic freezer bags; freeze 4 hours.
3. Remove bags from freezer 30 minutes before serving. Squeeze 1 bag with hands to break mixture into chunks; pour into a pitcher, and stir in 2 bottles club soda until slushy. Repeat with remaining bag, if desired. Garnish, if desired.