This represents a basic idea or concept behind how we could add those attending single's mixers to mail chimp.

jm-woo-chimp.php

<?php
/**
 * This is a simple snippet showing a hypothetical
 * solution for adding users to a mailing list in 
 * mailchimp.
 *
 * @author Jonathon McDonald <jmcdonald.ut@gmail.com>
 */

add_action('init', 'jm_add_actions')

/**
 * This will add all the actions once WordPress
 * has initialized.  
 */
function jm_add_actions()
{
  add_action('woocommerce_checkout_order_processed', 'jm_has_single_event', 10, 2)
}

/**
 * This checks if the user who just checked out
 * signed up for a single's event
 *
 * @param order_id Represents the ID of the order
 * @param posted Represents the contents posted...(not useful)
 */
function jm_has_single_event( $order_id, $posted )
{
	$current_order = new WC_Order( $order_id );
	$checkout_items = $current_order->get_items();

	foreach( $checkout_items as $item )
	{
		$product_categories = $item->get_categories();

		if( contains( $product_categories, 'singles-events' )
			// Add to mail chimp...
	}
}