How to create custom opt-in email for webforms
You can daisy-chain webforms using checksums, so on form 1 you set up your fields to collect contact detail data, then set up a second webform which actually adds them to your private email group, and having set the second webform up you can find the url with checksum on the CiviCRM tab of the webform, and then set that as the link that goes out in the email from the first webform.