Add permissions Tasks.Read.All or Tasks.ReadWrite.All
Currently it's not possible to access the tasks of all an organisation members regardless of who is connected to the application. At the moment, you can only access Outlook Tasks using delegated permissions (i.e. on behalf of the current user). You can reach outside their own tasks using the Tasks.Read.Shared scope but again, this only provide additional access to Outlook Tasks that were shared with the current user.
This would be a great add to the Microsoft Graph API to help Managers to follow the progession of their projects with new permissions like Tasks.Read.All or Tasks.readWrite.All.