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.
Best Answer
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.
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:
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.