Hi Marika,
According to my research, pleasae follow the steps in the articel below to enable external users to book Exchange room calendars.
Enabling External Users to Book Exchange Room Calendars.(Please Note: Since the web site is not hosted by Microsoft, the link may change without
notice. Microsoft does not guarantee the accuracy of this information.)
And this
article for you to set external users can see if the resource is already booked.
Hope this can be helpful.
Regards,
Aidan Wang
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