Email Contact Form That checks existing users

Hello all,

I know this may be a pretty easy script to run but I am wondering how I can create an email contact form that checks for existing users.

<?php
$email = 'myemail@example.com';
             if ( email_exists($email) )
                     echo "That E-mail is registered to user number " . email_exists($email);
  	     else
                     echo "That E-mail doesn't belong to any registered users on this site";
?>

I am using a free Woothemes template called Wootique. I am wondering how I can set the $email var to be existing users whatever function that may be in wootique. Thanks,

-craig