Basic jQuery

<!DOCTYPE html>
<html>

  <head>
    <meta charset="utf-8" />
    <title></title>
    <link rel="stylesheet" href="style.css" />
    <script data-require="jquery" data-semver="2.1.1" src="//cdnjs.cloudflare.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
    <script src="script.js"></script>
  </head>

  <body>
<div>
    <input type="checkbox" id="allow_max_members" checked='true'/>
    <input id="max_members" type="text" />
</div>
  </body>

</html>

$(document).ready(function () {
    $("#allow_max_members").bind('click', function() {
        var shown;
        shown = $(this).prop('checked');
        $("#max_members").toggle(shown);
    });
});

/* Put your css in here */

h1 {
  color: red;
}