The checkbox is always greyed out, I believe this has always been that
way. This is due to the reason that Google handles invitations
differently. Unfortunately, I have found that support via API is not
really complete. If you cant wait, there are a few other things you
can try out, but ymmv and you might accidentally spam attendees:
* calendar.google.sendEventNotifications = true (always sends
invitations on updates and additions)
* Modify the source: calGoogleCalendar.js, get sendItipInvitations to
return true. This will enable the checkbox, but I don't know what the
result will be.
Philipp