We have several computers/iPads/iPhones at home using IMAP mail service from our own domain. We currently share the addressbook between my wife and I, and have 4 email addresses between us having one main address card.
Since we share the addressbook and Apple ID, we also share the same iCloud calendar. For some reason, when I send a calendar invite, the invite never receives email notification and I have noticed that there is a grey question mark by the user.
Not really sure what this means but all data is basically in the cloud as we don't utilize any local calendars anymore. (This computer)