You can opt out, or leave, any Writing Group you've been placed in at any time. To do so:
- Go to your "Groups" page under the "My NaNoWriMo" tab.
- Click on the name of your Writing Group.
- Then, click on the button on the bottom right that says "Leave Group".
If you are the creator of a Writing Group, you won't be able to delete the group until all the other members have left.