<?php

function mailchimp_signup( $fields ) {
    $newsletter = $fields['custom_fields']['newsletter'];
    $name = $fields['firstname'];
    $email = $fields['email'];
    if($newsletter) {
       MailChimp->AddEmail($email, $name);
    }
}
add_action( 'booknetic_booking_done', 'mailchimp_signup', 30, 2 );