This should add a user to a mailchimp list when their user is created or approved in Discourse.
The client that I developed this for no longer uses Discourse, so I don't have means (or much desire) to test it.
I'll welcome PRs, but will likely not do much in the way of maintenance for free.
See https://mailchimp.com/developer/guides/an-introduction-to-rest/ for descriptions of the settings.
The <dc> part of the URL corresponds to the data center for your account. For example, if the last part of your Mailchimp API key is us6, all API endpoints for your account are available at https://us6.api.mailchimp.com/3.0/.
MIT.