How to add a category and attach it to the post on ACF save_post

wp_acf_add_cat_on_save.php

<?php
/*
 * Example of hooking into the Advanced Custom Field save process
 * to add a category to the taxonomy and attach it to the post.
 * This particular example retrieves the value from a custom field
 * named 'group_slug' and uses it as a category for the post.
 */
add_filter('acf/save_post',
  // Using a closure here but could be a separate function
  function($post_id) {
    // get group slug
    $group_slug = get_field('group_slug', $post_id);
    // Make sure we have a group slug 
    if( $group_slug && strlen($group_slug) > 0 ) {
        // Save group slug as a child category to the parent category that has an id of 8
        $arg = array('description' => "Group slug", 'parent' => 8);
        $group_cat_id = wp_insert_term(trim($group_slug), "category", $arg);
        // Add the group slug category to the post
        wp_set_object_terms( $post_id, $group_slug, 'category', true );
    } 
  },
  20 //setting to priority of 20 makes sure this isn't done until after the record is saved
);

?>