Either use template conditionals, or Javascript.
Back to using template conditionals. I use that css trick a lot on my own boards, but it is no good for not loading resources. The template conditionals plugin will only load the data etc if the usergroup is correct, so that is better if trying to not waste resources.
This would be pretty easy to do with jQuery (Make sure your theme has jQuery included). D: Just put this at the top of the header_welcomeblock_member template for an example.
<script type="text/javascript">
jQuery.noConflict();
jQuery(function($) {
$('body').append($('.imAwesome'));
});
</script>
Now if you look at your source as a member you should see <div class="imAwesome"></div> but not as a guest. Untested.