I do not see what the actual MailChimp Plugin does other than somehow connect to their service. Using the plugin might in fact be the answer, but here is what I would like to do.
I have a contact form created for my website. I also have the MailChimp service and a simple newsletter signup form (sorry, not using GF—it's just their html form) on every page. I would like to simply add a tick box on the contact form that the user can select if they want to opt-in to the newsletter list. In theory their information would feed directly to the MailChimp database.
Just for clarification, I don't have a site that gets many visitors. It would be customers or potential customers in our area only. If I receive more than 3 or 4 contacts a week through the form then its "really busy". Transferring the name over manually is an actual option. I'm just looking for the most efficient, secure, and economical way of doing this.