We'll create fresh WordPress site with Subscribe Google Groups installed. You have 20 minutes to test the plugin after that site we'll be deleted.
This plugin adds new widget ‘Subscribe Google Groups’, which allows visitors to sign up to admin-specified Google Group.
After user as entered her e-mail address and submits the form, plugin (tries to) sends e-mail to [email protected]. Afterwards, Google Groups sends confirmation link to submitter, and when user has clicked the link, she gets added to the group.
Note: If user domain has strict SPF/DKIM policies for e-mail handling, @googlegroups.com might reject their subscription request. In this case user needs to subscribe manually (by sending mail from her e-mail client or by visiting group page.)
Site can have 1-N widgets, each with their own configuration.