How to list users of particular applications created under one QB account?
If you have several applications created under the same QB account and you wish to list users belonging to a particular application (not all users under your account), you can use tags.
Just set tag when registering new user and then retrieve your users by tags.
Users can be created through your QB admin panel: Applications> Your App> Users> Add New User.
Here you can read how to retrieve users by tags.