How does Zulu handle invitations?
Unfortunately, we aren't currently capturing any invitation data in Zulu/FileMaker, just the fields that are currently mapped. If you try to send an invitation from iCal, you'll get an error saying this isn't supported on this calendar.
Google is a little different, as it's a separate independent calendar on Google, so you can accept invitations there, but there's no way (that we know of) to specify one of the Zulu calendars as the default, it will just want to add it to the primary calendar associated with the account. You then have to basically duplicate the event to the zulu calendar...so not really ready for prime time.
This is something we're looking at, but it adds a lot of complexity to the process, so no firm plans yet.
In iCal, however, you can elect which calendar to add an accepted invitation to, so those records will end up in FileMaker. This is also the case with Apple's data recognition-- where you can highlight a date in an email and create an iCal entry for it--you can specify a Zulu/FileMaker calendar there as well.