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We are using Office 365 but the meeting room is a shared resource between multiple companies with different policies, so we need to support Apple iCal, Gmail etc.

As far as I have understood, publishing a calendar will not let users edit it (or at least make new appointments/edit their own appointments)? In this case it's meaningless, since everyone has a right to book the meeting room, regardless of whether you are part of our organization or not.

One option seems to be handing out guest access to the shared resource mailbox using our Azure AD, but when I try, I can't give editing permissions to calendars even to the guest users?

Perhaps using Office 365 in this case is pointless? It seems designed for large corporations, not smaller companies working tightly and sharing resources with other small companies.

  • I worked for a while at a place that wanted to share an Exchange Online calendar outside the organization. I never figured out how to do it and I suspect it's not something Microsoft expects people to want to do. You can publish a calendar to Sharepoint Online from Exchange but if I remember correctly it doesn't update dynamically and no it doesn't allow people viewing the calendar in SPO to book appointments. We considered a third-party cloud-based room-booking app instead. – Todd Wilcox Oct 31 '17 at 19:56
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By default a room mailbox does not permit external users to make bookings. However there is an option that you can configure to allow external people to make bookings if you need them to. It's the ProcessExternalMeetingMessages flag.

Get-Mailbox "external room" | Get-CalendarProcessing | Select *external*

If an external sender attempts to book the meeting room they will not receive an acceptance or rejection message, which may lead to confusion if the room is assumed to have been successfully booked (given that the ressource hat automatic processing anabled).

To configure the room mailbox to process external requests:

Get-Mailbox "external room" | Set-CalendarProcessing -ProcessExternalMeetingMessages $true

This change will enable the processing of external meeting requests. The meeting request, however, is still subject to being accepted/rejected based on availability of the room and any other booking policies.

Note (fooled myself with this one a few times): This change will only take effect for new requests. Any meeting requests from external senders that were received before the setting was enabled will not be processed.

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    I have tried this, but I still can't add guest users to the mailbox (to get access to the shared calendar). I can publish the calendar and add it in my external test client, but it shows no bookings, and I can't book anything either... confused – Jay Oct 31 '17 at 14:55
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    Ah, publish the calendar for everyone to see, then let people book appointments with the room as participant... and the booking will be accepted automatically. I get it now, and it works! – Jay Nov 1 '17 at 15:51

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