Battlepass/ leaderboards system
The app should add an element of gamification. By making the app more fun to engage with on the client side we could have better retention overall.
A Leader board system with an opt-in / out function on the client side (within the groups) would be the optimal way to implement this feature. Then each client can activate / share their numbers and metrics with the groups they choose. If they don't opt into it they don't have to see others scores. This would allow the competitive clients to push themselves and eachother while not forcing the program onto those who don't want to compete.
BattlePass system this could be easily integrated where clients earn tokens for challenges based on complience, and refferals. Those tokens could hold a value that the trainers could redeem for different prizes. The net benefit is that the clients get incentive zed to use the app and refer friends to the business running the battle passes.