fitness challenge-on-line or together

Has anyone created a on-line fitness challenge that their clients can do together? I have a couple 1-1 clients that want to try to form a fun competition against eachother. Most have never met. They thought it would be a good way to have support. One client bought the fitbit which is great! She gave me here log in information so I can keep track of her and give her tips where ever I see is needed. She was hoping other clients of mine would buy it and they can have discussions with eachother.

I was trying to think of a way I can create a challenge with my current clients and a way to gain new clients. I am in-home trainer and rentn a martial arts studio once a week. Maybe the challenge can be completly on-line or I can have it as some package deal were they train with me, get the fitbit so I can also do nutrition based on calories burned. Also, the fitbit tracks steps taken so we can have a steps challenge. Every week have weighins. Maybe everyone can also put an extra $10.00 as an additional incentive. The top percent loss can win that money. So there are many options that I can do. I just want to see if anyone can help me organize this idea to meet folks I am working with and some people that I can reach out to that want to do it on thier own.

How much do I charge ETC.

Also, do you know if I promote a product to clients or on my website do I get endorsements for that? I heard I could. Not sure how.