Has anyone managed to use the very hand UserForms module: https://github.com/silverstripe/silverstripe-userforms and extended it to include a checkbox (or similar) allowing users to be optionally added to a MailChimp mailing list? http://www.mailchimp.com
The developer says:
"Simply use the DataExtension on SubmittedForm and hook into updateAfterProcess to perform any actions on after submission."
but this is out of my knowledge range...
basically working but needs a lot more love - just go ahead and PR :) https://github.com/lerni/EditableMailchimpSubscriptionField