Currently Being ModeratedOct 3, 2013 2:31 PM (in response to chrisfromguildford)
I am having this problem too. I cannot dismiss the alert, and I can't even quit Apple Calendar. Neither "Remove Group" nor "Edit Event" resolves the issue, and the event no longer has any invitees listed.
I am trying to copy all of my local calendars into iCloud by exporting each one individually and importing them back into Apple Calendar.
Really angry at this point. It's been years and it still seems like there's no good way to sync your contacts and calendars across devices and services. This should be insanely simple...