Once your group hits its tipping point, click "Collect payment."
If you don't see "Collect payment," see this article.
Next, you'll be taken to a page to look over the details of your Pay By Group one final time. This next page is also where you'll put in your bank account information.
Once you've confirmed the details and entered your bank account information, click the "Collect payment" button to complete your Pay By Group. You're all done!