Which version of S2Member are you running. The next release which I am beta testing has fixed all of the previous Mailchimp issues.
If it's ver. 3.5.8 then hang fire for a couple of days until Jason can get this new version finalised. I have thoroughly tested it and it is flawless with Mailchimp.
The version you currently have should still be adding names to your list as long as you have the API key and List ID for the free subscribers set.
Are you receiving the list 'opt in' email that get's sent when a new user registers?
Before they can be added to any list they must click on the confirmation link in the email they will be sent.
Let me know, as there are also a couple of server configurations which might be stopping your site communicating with Mailchimp.
If new signups are receiving the opt in email, clicking the link and still not being added then we'll look at each step and work out why not.
Ross
