Update: This issue should now be fixed - thanks for your patience! You can now invite Pinners to group boards.
Sorry for the trouble!
We're having some issues with group board invites right now and our team is looking into it. If you're seeing an error message telling you confirm your email to invite people, don't worry - your email address is confirmed and it's an error on our end!
We're doing what we can to get group board invites back up and running, and we appreciate your patience with us in the meantime.