It really depends how you have your database set up. If you share some screenshots of what you have that might help. Let me try to set up an example for you to see one way you can set up the database structure and I will post the link back here soon.
Here is just one way to do this. I am not sure how you have everything else setup but instead of grouping, you could display your list of users depending on what you need or who has access to these users.
My list of user is on my app data, located in “data” section.
What type of constraint should I do? Based on user ?
When I preview the app, I want to see average stats coming from different user that I have in my database.
So, if log in with two diferent users it should be different from the other
So the best thing would be to set up your privacy rules for Stats. Something like Current user is this Stat’s creator (unless you have a user field besides creator that you are using). Or something like that should fix it.