Ninja Forms - Mailchimp plugin
From GLMWiki
All forms going to mailchimp need to have the following fields and they need to be required. First Name Last Name Email
Setting Mailchimp API Key in Forms > Settings > Mailchimp This is found in your MailChimp "Extras" setting and looks like this: xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx-xxx
Each form also might need to have the Double Opt in checked in form settings
