<div class="users form">
<?php
    echo $this->Form->create('User', array (
            'type' => 'post',
            'inputDefaults' => array (
                'div' => false
            )
        )
    );
?>
<script>
    $(document).ready(function() {
        $('#UserFirstName').focus();
    });
</script>
<fieldset>
    <legend></legend>
    <h2>Registration</h2>
    <?php
        echo $this->Form->input('firstName');
        echo '<div class='clear'></div>';
        echo $this->Form->input('lastName');
        echo '<div class='clear'></div>';
        echo $this->Form->input('username');
        echo '<div class='clear'></div>';
        echo $this->Form->input('password', array ('class' => 'short'));
        echo '<div class='clear'></div>';
        echo $this->Form->input('password_confirm', array('type' => 'password', 'label' => 'Confirm Password:   ', 'class' => 'short'));
        echo '<div class='clear'></div>';
        echo $this->Form->input('email', array('label' => 'Email:   ', 'default' => $email));
        echo '<div class='clear'></div>';
        echo $this->Form->input('id', array('label' => 'id: ', 'type' => 'hidden', 'default' => 37));
        $qmark = $this->Html->image('qmark.png', array('height' => 15));
        echo '<div class='clear'></div>';
        echo $this->Form->input('number', array('label' => 'Number:', 'class' => 'short', 'after' => $qmark));
        echo '<div class='clear'></div>';
    ?>

echo $this->Form->input('number', array('label' => 'Number:', 'class' => 'short', 'after' => $qmark));

<input type="button" id="btnSignUp" value="SignUp" />
<div id="popup">
  <div id="popupinfo">
    <p> Now, your registration form has been loaded...! </p>
    <p> You have field your required fields and submit form.</p>
    <p class="button"><a href="javascript:void(0);">OK</a></p>
  </div>
</div>
<div id="registerDiv">
  <p class="required">Fields with Mark (*) are required fields </p>
  <form name="frmregister" id="frmregister" method="post" action="#registerDiv">
    <table cellpadding="0" cellspacing="0" class="table" border="0">
      <tr>
        <td>*Name:</td>
        <td><input type="text" class="input" size="17" /></td>
      </tr>
      <tr>
        <td>*Username:</td>
        <td><input type="text" class="input" size="17" /></td>
      </tr>
      <tr>
        <td>*Password:</td>
        <td><input type="password" class="input" size="17" /></td>
      </tr>
      <tr>
        <td>*Confirm Password:</td>
        <td><input type="password" class="input" size="17" /></td>
      </tr>
      <tr>
        <td>Email:</td>
        <td><input type="text" class="input" size="17" /></td>
      </tr>
      <tr>
        <td colspan="2">&nbsp;</td>
      </tr>
       <tr>
         <td colspan="2" align="center"><input type="submit" value="Register" id="btnsubmit" /></td>
       </tr>
    </table>
  </form>
</div>

.input{
  border:1px solid #333;
}
.required{
  color:red;
  font-size:12px;
}

#registerDiv{
  display:none;
  margin-top:10px;
  margin-left:20px;
  border:2px solid #333;
  padding:3px;
  background:#cdcdcd;
  width:280px;
  text-align:center;
}
#popup{
  display:none;
  padding:10px;
  background:#969696;
  position:absolute;
  top:25%;
  left:25%;
  z-index: 99999;
  opacity:100%;

}
#popupinfo{
  padding:2px;
  text-align:center;
  background:#cfcfcf;
}
#popupinfo p{
  font-size:14px;
}

#popupinfo .button {
  text-align:center;
}
#popupinfo .button a{
  text-decoration:none;
  border:1px solid #333;
  width:20px;
  padding:5px;
  background:#1A1A1A;
  color:#eee;
}

#popupinfo .button a:hover{
  background:#eee;
  color:#1a1a1a;
}

#mask{

  background: #000;
  position: fixed;
  left: 0;
  top: 0;
  z-index: 10;
  width: 100%;
  height: 100%;
  opacity: 0.8;
  z-index: 999;

}

$(function() {
    $("#btnSignUp").click(function() {
        $("#registerDiv").fadeIn(300);
        $("body").append("<div id='mask'></div>");
        $("#mask").fadeIn(300);
        $("#popup").fadeIn(300);
    });
    $("#popup .button a").click(function() {
        $("#popup").fadeOut(300, function() {
            $("#mask").remove();
        });
        $("#frmregister").find("input:first").focus();
    });
});

$(document).ready(function() {

       $('input[name="number"]').click(function() {

          alert('Display your popup here...'); // maybe you should use another js library for popup
      });
    });

$('input[name="number"]').click(function() {
    // Your code here

}

<a href="#"></a>

$("#button").click(function(e){
    e.preventDefault();
    $("#popup").show();
});

visible:none;