Example of using the pmpro_mailchimp_listsubscribe_fields filter to send extra fields to Mailchimp. The fields must be created in Mailchimp first, or you must used the pmpro_mailchimp_merge_fields filter to create them.

my_pmpro_mailchimp_listsubscribe_fields.php

<?php
/**
 * Sync additional user fields to Mailchimp.
 * You must create the fields in Mailchimp first.
 * Or, you can use the `pmpro_mailchimp_merge_fields` filter to create them through the API.
 *
 * You can add this recipe to your site by creating a custom plugin
 * or using the Code Snippets plugin available for free in the WordPress repository.
 * Read this companion article for step-by-step directions on either method.
 * https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 *
 */
function my_pmpro_mailchimp_listsubscribe_fields( $fields, $user ) {
	$new_fields =  array(
		"TITLE" => $user->title, 
		"COMPANY" => $user->company, 
		"ADDRESS" => $user->address, 
		"CITY" => $user->city,
		"STATE" => $user->state,
		"ZIPCODE" => $user->zipcode,
		"COUNTY" => $user->county,
		"REGION" => $user->region,
		"INDUSTRY" => $user->industry,
		"PHONE" => $user->phone
	);

	$fields = array_merge( $fields, $new_fields );
	  
	return $fields;
}
add_action( 'pmpro_mailchimp_listsubscribe_fields', 'my_pmpro_mailchimp_listsubscribe_fields', 10, 2 );

/**
 * Tell PMPro MailChimp to always synchronize user profile updates. By default it only synchronizes if the user's email has changed (optional).
 * Requires PMPro Mailchimp v2.0.3 or higher.
 */
add_filter( 'pmpromc_profile_update', '__return_true' );

We've blogged about using this recipe in the "Send Additional User Information Fields to MailChimp" article here.