I am still not getting the contact forwarded to MailChimp when they create an account. Looking further, I see that the line I had to add:

if ((int)$newsletter == 1) {
mailchimp_add($email_address);
}

only refers to the email address, but in my MC, for other integration, I have the first name as mandatory. Is it possible that because it only forwards the email address and NOT the first name, that MC simply does not recognize it or accept it?

I would need to add the first name to this, so how would I edit it?