Is there a unique ID for each user? If so, how can I display this on-site?

Yes. Using an s2Member shortcode you can do this inside a Post/Page.

[s2Get user_field="ID" /]

See also: <> If you install a plugin like ezPHP, you can use PHP code to get the user's ID.

<?php echo esc_html(get_current_user_id()); ?>