To see which groups each Trainer/Manager controls go to the Manage Users page and click on the name of the Trainer or Manager. A right panel window with the Trainer/Manager information will open, click on the Permissions tab.
To see who has access to a specific group go to the Manage Users page and click the Set Up button at the top, right and choose Groups from the dropdown.
Once you are on the Groups page select a group on the left. On the right you will see which Trainers/Managers have access to all Trainees, as well as which Trainer/Managers are restricted to this specific group.