gf-user-list.php

<?php
/**
 * Route the notification to multiple users in a bcc field.
 * 
 * @see https://joshuadnelson.com/user-dropdown-list-custom-notification-routing-gravity-forms/#comment-12376
 * @see https://www.gravityhelp.com/documentation/article/notification/
 */

// NOTE: This is untested code, please use with caution. Test and improve as necessary.

add_filter( 'gform_notification_3', 'route_user_email_notification', 10, 3 );
function route_user_email_notification( $notification, $form , $entry ) {
	// Creat an array to store all the user ids from the checkbox field. Initializing it here in case it's not filled.
	$bcc_users_array = array();
	
	foreach( $form['fields'] as &$field ) {
		// Find the right field
		if( $field['type'] != 'checkbox' || strpos($field['cssClass'], 'user-emails') === false )
			continue;
		
		// Pull out the field id and the $entry element
		$field_id = (string) $field['id'];
		
		// ------- Since your field is a checkbox, this should return an array, correct?
		// I'm not 100% sure on the output here, so you might need to debug and modify to build an array of user ids
		$bcc_users_array = $entry[ $field_id ];
	}
	
	// Make sure we have an array with values in it
	if( !empty( $bcc_users_array ) && is_array( $bcc_users_array ) ) {
		
		// Initilize this value as an empty string, then we can add to it.
		$notification[ 'bcc' ] = '';
		
		// Cycle through the users, assuming the values are all user ids
		foreach( $bcc_users_array as $bcc_user_id ) {
			
			// Make sure the user id is a valid int, otherwise skip it.
			if( !absint( $bcc_user_id ) ) 
				continue;
			
			// Grab the email address from the user id, add it to the bcc field	
			$email = get_the_author_meta( 'user_email', $bcc_user_id );
			if ( is_email( $email ) ) {
				$notification[ 'bcc' ] .= $email . ',';
			}
		}
		
		// Remove the trailing comma
		$notification[ 'bcc' ] = rtrim( $notification[ 'bcc' ], "," );
	}
	
	return $notification;
}