Allow admin people or custom visitor IP to visit the site, otherwise, people are redirected to a specific page such as coming-soon.html Code to copy/past in functions.php in theme folder.

functions.php

<?php
/**
 * REDIRECT TO COMING SOON PAGE FOR NOT ALLOWED VISITORS
 * Allow admin people or custom visitor IP to visit the site, otherwise,
 * people are redirected to a specific page such as coming-soon.html
 * Code to copy/past in functions.php in theme folder.
 */

// Disallow access to website if not admin:
remove_filter('template_redirect','redirect_canonical');

if ( ! ( is_super_admin() || is_login_page() || $_SERVER['REMOTE_ADDR'] == '000.000.000.000' ) ) {
  header( 'Location: http://www.my-wordpress-site.com/coming-soon.html' );
	exit();
}

function is_login_page() {
    return in_array( $GLOBALS['pagenow'], array( 'wp-login.php', 'wp-register.php' ) );
}
?>