justin_h83,
Sorry-- I missed your question...usually it's best to start a new topic when you have a new question, that way it will be noticed.
OK, for your first question, this is a bit of a tricky one because you'll have to check the database for the username. Off the top of my head, you'd do something like this in your template:
<?php
global $wpdb;
$current_user = $wpdb->get_var("SELECT COUNT(*) FROM wp_participants_database WHERE username = '$post->post_name'");
if ($current_user == 0)
echo do_shortcode('[pdb_signup template="bootstrap"]');
?>It probably won't work exactly like that, you have to change the field names to what you have in your database, but you get the idea.
Now, to make an "unregister" probably the best thing to do is give them a checkbox to uncheck if they want to unregister (like you would do for a mailing list opt-in) and then make sure to filter your lists so such records are excluded. No point in trying to figure out how to make it possible for them to actually delete a record.