In the authenticate user server behavior you have an option to save one or more fields from the users table as Session variables. Just choose the email field to be stored in the session and then you can display it on any page after they have logged in like:
<?php echo(isset($_SESSION['UserEmail'])?$_SESSION['UserEmail']:""); ?>
(this code assumes you saved it in the session as "UserEmail" but you have a chance to set it to whatever you want in the UI)