<?php
define('BUYER_GROUP_ID',111); //zamiast 111 wstawić ID grupy
function assign_buyer_to_group(int $order_id) {
    $order = wc_get_order($order_id);
    if(!($order instanceof \WC_Order)) { return; }
    $user_id = $order->get_customer_id();
    wp_set_object_terms($user_id, BUYER_GROUP_ID , 'user-group', true);
}



//add_action( 'woocommerce_order_status_completed','assign_buyer_to_group',0,1);
add_action( 'woocommerce_new_order','assign_buyer_to_group',0,1);