It doesn't make sense for the Register Mailchimp button to be within the section for MailChimp List for Registrants. The MailChimp List for Registrants is meant to funnel in emails of people who registered for the webinar, the Register MailChimp button is now registering the same people who already registered? Infinite loop. This button should have it's own section because the Invitation list is most likely the people who you are not going to manually register and it shouldn't be in the Registrants list because the button relies on a Source not a Destination.