Help develop a service for personal trainers and dietitians

I’m currently in the process of developing a service to help personal trainers, and dietitians work together to track their clients progress. e.g. Progress reports, Body measurements, ‘Before and after’ photos, Workout or nutrition records , Communication, and maybe payments since I will focus mainly to fitness aligned a common goal sports or fitness.

what kind of incentive should I give for user interview regarding to Fitness App?

note: I need to extract information about thinking, feeling, attitudes, needs, and wants, and tried to understand their emotions .

The Approach: A mobile app that helps people track diet and workout.

Goals:

• Develop a research-informed understanding of who, how and why wants to be more physically fit.
• Design a product that improves the user experience of becoming more physically fit.
• person can invite personal trainers and dietitians to their data in order to continue their objectives
• improve workflow between personal trainers and dietitians

My current strategies

surveys in industry forums (reddit fitness ) - didn't work
Facebook groups ( dietitians , nutritionists ) . 2 people interested , but too low
university alumni network ( I haven't tried this )