// Get the user info
$user = JFactory::getUser();

// If the user is not registered (aka a guest), show some message, otherwise show the item's content
if ($user->guest) {
    echo '<p>You need to be registered to view this content.</p><p>You can <a href="#">sign up here</a>.</p>';
} else {
?>

<!-- the actual code for item.php continues -->

<?php
} // Here we close the if/else statement which controls what the user sees (this will be at the bottom of the item.php file)