//Following code adds a customer as a amelia customer if they purchase a product with a given product id
add_action( 'woocommerce_order_status_completed', 'register_student_amelia', 10, 1 );

//Checks if the relevant product is being purchased to update customer
function register_student_amelia_and_year_group($order_id){
    $wc_product_id = 4623; //replace with your product id
    $order = new WC_Order($order_id);
    $items = $order->get_items();
    foreach($items as $item){
        if ($wc_product_id == $item['product_id']) {
            add_user_as_amelia_customer($order);
        }
    }
}

function add_user_as_amelia_customer($order){
    global $wpdb;
    $user_id = $order->customer_id;
    $user = new WP_User( $user_id );
    $user_info = get_userdata($user_id);
    $user_email = $user_info->user_email;
    $first_name = get_user_meta( $user_id, 'first_name', true);
    $last_name = get_user_meta( $user_id, 'last_name', true);
    $phone_number = $order->get_billing_phone();
    if(!in_array('wpamelia-customer',$user->roles)){
    //Add amelia customer
        $user -> add_role( 'wpamelia-customer' );
        $wpdb -> insert('wphx_amelia_users', array( //wphx_amelia_users was my table name but you need to navigate to MySQL database in you cPanel to see what your amelia users table name is
        'firstName' => $first_name,
        'lastName' => $last_name,
        'email' => $user_email,
        'phone' => $phone_number,
        'type' => 'customer',
        'externalId' => $user_id,
        'note' => NULL
        ), array('%s','%s','%s','%s','%s','%d','%s','%s'));
    }
}