Enabling the Google Groups application will automatically add your submitters’ emails to your Google Group. Type the name of your group and click Visit to make sure it’s the right one. Then select, in the dropdown list, the form field which contains your submitters’ email addresses and press Save.
The form users will receive via email the invite link to join your group.
If you encounter any issues with your group, make sure to check this article from Google. It contains the most common issues and their solutions.