function maybe_add_existing_user_to_blog() {
	if ( false === strpos( $_SERVER['REQUEST_URI'], '/newbloguser/' ) ) {
		return;
	}

	$parts = explode( '/', $_SERVER['REQUEST_URI'] );
	$key   = array_pop( $parts );

	if ( $key == '' ) {
		$key = array_pop( $parts );
	}

	$details = get_option( 'new_user_' . $key );
	if ( ! empty( $details ) ) {
		delete_option( 'new_user_' . $key );
	}

	if ( empty( $details ) || is_wp_error( add_existing_user_to_blog( $details ) ) ) {
		wp_die(
			sprintf(
				/* translators: %s: Home URL. */
				__( 'An error occurred adding you to this site. Back to the <a href="%s">homepage</a>.' ),
				home_url()
			)
		);
	}

	wp_die(
		sprintf(
			/* translators: 1: Home URL, 2: Admin URL. */
			__( 'You have been added to this site. Please visit the <a href="%1$s">homepage</a> or <a href="%2$s">log in</a> using your username and password.' ),
			home_url(),
			admin_url()
		),
		__( 'WordPress &rsaquo; Success' ),
		array( 'response' => 200 )
	);
}