Assigning dummy services providers to more than one user

Hi - I'm trying to create multiple dummy users per this thread, but it's not working. I'm running v1.4 and when I include the filter in that thread it doesn't work. Here are the steps I used:

- created a new User in Users > Add New
- In Appointments > Settings > Service Providers added a new provider and assigned the just-created user (user id = 21 in both places).
- If I checked 'dummy' next to the provider, the email went to my default dummy account as set up in Settings; if I unchecked 'dummy' it went to the user email attached to user 21.

Here's the code in my functions.php:

function modify_dummy_email( $email, $worker_id ) {
  if ( 21 == $worker_id )
  $email = '';
  return email;
add_filter( 'app_dummy_provider', 'modify_dummy_email', 10, 2 );

I have multiple users who each provide several services, and the services have different schedules, some overlapping (the users here are like room monitors and can monitor several services simultaneously). So I need user1 to be notified of bookings for service1 and service2 and user2 to be notified for service3, service4 and service5.

Ideally the users wouldn't have to log into five different user accounts to view what's been booked and confirm/update appointments... is this possible? (I'd really love it if I didn't need to continue updating the functions.php file every time the client adds services/providers, too, but I'll take what I can get). Thanks!