Just went through the same thing, my understanding of this issue.
1. When this addon adds your event to Google Calendar you are recorded
as the owner of the event.. There is currently no way to sync the
event to Google Calendar while setting organizer to be someone else
(ie the guy who sent you the invitation)
2. As a result Google Calendar was sending emails to all the
participants/attendees for these events which you added by accepting
email invitation.
3. Many people posted horror stories of accepting invitation to a
company wide email (with 100+ participants) and having Google Calendar
then Spam all the participants with duplicated invitations.
Until all of the above is sorted out, as a workaround this addon does
not allow adding events by responding to email invitations.. There is
yet another workaround to prevent the original problem of Google
Calendar spamming the attendees with invitations, and for that there
is a new setting which prevents this addon from sending attendee/
participants info to Google Calendar, ie all events accepted via email
will have no participants, thus preventing Google from spamming them.
There is a way to enable responding to email invitations, I will show
it below. If you enable it, you will also need to enable the other
setting of not sending attendee info to Google Calendar, otherwise
you'll end up with the same horror story of Google Calendar re-sending
email invitations to all attendees.
How to enable it:
In thunderbird go to Tools/Options/Advanced/Config Editor
Change the following settings:
calendar.google.enableAttendees false --- do not send
attendees to Google
calendar.google.enableEmailInvitations true --- allow
accepting invitations via email
calendar.google.sendEventNotifications false --- not sure what
this does, I set it to false to be safe
After changing the above setting, switch to Calendar tab in the left
pane (where you have Calendar/Mail/Tasks buttons) then right click on
your Google Calendar, and select proprties. There will be additional
setting there for EMail:. Set the EMail to the email address to which
invitations are being sent (ie your thunderbird email account)
After that you should be able to accept invitations and they will sync
to your Google Calendar. For me accepting reschedules did not work, I
had posted a patch that fixes that.