Yes just worked this one out.
First add the check box field to your mailchimp list
so go to 'list fields and merge tags' under settings:
set up your radio button as required (1) and then copy the merge tag it outputs for this (2):
back in webflow paste this merge tag into the radio button field settings for GROUP NAME.
If you have more than one radio button then they all get the same group name merge tag and the user can only select one of them. If you want them to be able to click more than one option then each will need to have a separate merge tag - so that will mean you need to set up several radio buttons in mailchimp first so they can all have a different merge tag.
Save, no need to do anything else as long as mailchimp is integrated already in the form field action box.
Test it and all the fields in the mailchimp list will be populated.