Comment 62 for bug 810622

So--had the same problem and here's how i solved it worked it out.

follow these directions to download plugins and create network google calendars for all of your calendars (based on your emails). So i have <email address hidden> and a <email address hidden> emails where those same emails are connected to google for calendars (and I'm not using gsuite) https://www.techrepublic.com/article/how-to-add-a-readwrite-google-calendar-to-thunderbirds-lightning/

THEN, create new local calendars for each email you have that are the same emails as the google calendar emails.

When you then receive invitations to those emails, you can accept invitations and they will go into your local lightning calendars but they will also sync with your google calendars. I have my google cals in lightning checked to see all events prior to this change, but new events don't require it.

whew!