If you want to share one or more resources with an external person you can create an iCal link and allow external bookings:
Create an iCal link:
To publish a room resource by iCal you need to have access to the Exchange online PowerShell module
You can use the following command to publish the resource for one year:
Adjust the identity with the Mail of the room.
Set-MailboxCalendarFolder -Identity room@myorganization.com:\calendar -PublishEnabled $true -DetailLevel Full -PublishDateRangeTo OneYear -PublishDateRangeFrom OneYear
To get the link of the share, you need the following command:
Get-MailboxCalendarFolder -Identity room@myorganization.com:\calendar
Once you have done this, you can share the iCal link with an external person. The external people can then invite the meeting room as attendee. The room will then accept the booking (if it is free) and the ROOMZ display will also show the appointment correctly.
For Outlook client users, if you receive an error when adding the calendar, please follow these steps; otherwise, the calendar will not be displayed:
In Outlook, select File >Account Settings >Account Settings. You will see a list of your email accounts.
Select the Microsoft Exchange account that you use to manage someone else's calendar and choose Change....
Choose More Settings, followed by the Advanced tab.
Uncheck the checkbox next to: Turn on shared calendar improvements, see the print screen below:
Restart Outlook and try again to add the calendar.