My personal trainer has an iphone and uses "Fitness Builder" but on top of the app cost which used to be $10 or something but actually in double checking the name of the app I guess it's free now. However, I know on top of just having the app, she pays a monthly fee for her subscription (pro I think) that lets her manage multiple clients. She can track body stats in there I think plus uses the builder thing to create workouts and can track what exercises they did at what weights. It does seem like a nice setup, but maybe overkill with that cost especially if you need it just to track peoples' weights.