My family drinks celery and carrot juice almost every day and has no idea, it is hidden in frozen fruit smoothies I make in our Vita-Mix.
I would estimate I use 1 cup OJ, 2.5 cups frozen Spectrum fruit blend (from Costco), handful of baby carrots or 1 med carrot, and 1 large stalk celery per batch (split into 3 portions for dh and kids). If the celery is particularly bitter, I add a squirt of honey.