Create user if one does not exist

// Create user if one does not exist
function tn_checkout_create_acct( $post_data ) {
    $user = get_user_by( 'email', $post_data['billing_email'] );
    if ( $user ) {
        $post_data['createaccount'] = 0;
    } else {
        $post_data['createaccount'] = 1;
    }
    return $post_data;
}

add_filter('woocommerce_checkout_posted_data', 'tn_checkout_create_acct');

// Attach order to existing account if user not logged in
function tn_checkout_set_customer_id( $current_user_id ) {
    if ( !$current_user_id ) {
        $user = get_user_by('email', $_POST['billing_email']);
        if ( $user ) {
            $current_user_id = $user->ID;
        }
    }
    return $current_user_id;
}

add_filter('woocommerce_checkout_customer_id', 'tn_checkout_set_customer_id');