Example of how you can add tags into the subscribe_args used during the MailChimp subscription call.

tw_ee_add_tags_on_mc_subscribe_call.php

<?php //Please do not include the opneing PHP tag if you already have one.

add_filter('FHEE__EE_MCI_Controller__mci_submit_to_mailchimp__subscribe_args', 'tw_ee_add_tags_on_mc_subscribe_call', 10, 3);
function tw_ee_add_tags_on_mc_subscribe_call( $subscribe_args, $registration, $EVT_ID ) {
	
	$subscribe_args['tags'] = array('Test 1', 'Test 2', 'Test 3');
		
	return $subscribe_args;
}