Thanks for posting! I'm afraid that the plugin does not provide the direct functionality to subscribe email addresses into a specific, hidden group.
In order to do this, the best bet would be to utilize the MailChimp embed code, along with a bit of customized code. Ultimately, the idea is to hide the form fields associated with your form's group fields, then pre-fill (or rather, pre-check) the values associated with the groups you want to add them into.
This MailChimp article introduces the concept of hiding and populating a hidden form field:
Can I set up my signup form so that I can see where my recipients signed up?: http://eepurl.com/SKbn
In order to apply this to groups, I'd recommend taking a look at the MailChimp embed code to see how the group fields work, which should help in determining how pre-checking group fields work. Once those group fields are pre-checked, you can then choose to hide them and thus have the hidden automation you're looking for.
If you have any questions, let me know!