Hi Danny,
I used your form generator to create the code:
<p>
<label>Which optional emails would you like to receive?</label>
<label>
<input name="GROUPINGS[13877][]" type="checkbox" value="33437" checked><span>Special Offers</span>
</label>
<label>
<input name="GROUPINGS[13877][]" type="checkbox" value="33441" checked><span>Newsletter</span>
</label>
</p>
…which matches the grouping in mailchimp.