Hi Again,
I see that WooCommerce provides a hook. So, If it is WooCommerce registration,
adding the following code to bp-custom.php

/**
 * Add user registered via WooCommerce to current network.
 * @param $user_id
 */
function buddydev_add_wc_user_to_network( $user_id ) {
	if ( ! function_exists( 'mnetwork_add_user' ) ) {
		return ;
	}

	$blog_id = get_current_blog_id();
	mnetwork_add_user( $user_id, $blog_id );

}

add_action( 'woocommerce_created_customer', 'buddydev_add_wc_user_to_network' );

And then registering a user will work. Can you please try that.