This is really great, Carl. Thanks for getting this out. A quick question, though, since I’m not able to test at the moment:
Mailchimp, for as great as it is, is really dumb about how they handle adding/removing people to multiple groups. What happens if someone is already on a list, and then they use a GF to add themselves to a group within that list? If you try to do this with a normal form, this will fail and serve an “already subscribed” error.
What happens if you do this with a GF? Is it different because it’s going through the API?