The Customer and Instructor on a reservation will always see alerts for their own flights, there are no permissions required. To view all Due Soon and Overdue Alerts a user will need permission.
Navigate to Settings>Users>Roles & Permissions, select a role to edit and scroll to the Dispatching & Flight Section.
Set up Due Soon and Overdue alerts
Navigate to Settings> Dispatch>Overdue Flight Alerts
From here you can turn on or off Due Soon and Overdue Alerts as well as define when to trigger the alerts
Subscribe to Alerts
Note: User must have View Due Soon/Overdue Alerts
Navigate to your User Profile>Account Settings> Email Notifications