The Community Builder (CB) plugin allows you to show a newsletter checkbox during the registration process on your site if you use CB for registrations. This plugin will show a checkbox and all the fields that you want your users to fill up in order to be subscribed for your newsletter. To enable the plugin navigate to your joomla backend -> components -> community builder -> plugin Manager -> CMC - Newsletter registration
This plugin is automatically installed with CMC if we detect CB during the installation. If you have installed CMC first and after that CB, then you'll have to install CMC again in order for the plugin to be installed. You don't need to uninstall first - you can just install the same version over the old one.
The fields that are required in mailchimp have to be present in the newsletter form, unless you've configured a field mapping. The standard field mapping for email, first name and last name looks like this:
EMAIL=email FNAME=name:0 LNAME=name:1