add to your functions.php file:
add_filter('wpmu_signup_user_notification', 'my_if_notification_required');
function my_if_notification_required( $user ){
$user = get_userdatabylogin( $user );
if( $user && in_array('editor', $user->roles) ) return true;
else return false;
}
add_filter( 'wpmu_signup_user_notification_email', 'my_notification_text');
function my_notification_text( $user, $user_email, $key, $meta ){
$user = get_userdatabylogin( $user );
if( !$user || !in_array('editor', $user->roles) ) return false;
$message = 'Your message';
return $message;
}