Microsoft 365

Sharing meeting rooms between two Office 365 tenants

If you have a conference room, company car, or equipment that everyone needs to use, then you need to come up with a way to make those resources reservable to everyone. This is easy if you have one Microsoft 365 tenant. Just follow the procedure on Microsoft Docs website.

Recently I was asked how can we share meeting rooms between two Office 365 tenants?

Here the things get a bit more complicated. There two ways you can achieve that. The first one is to enable free/busy calendar sharing between the two organizations and allow external bookings for the meeting rooms and the second is to create meeting rooms in both organizations and then setup forwarding between rooms.

Sharing rooms by enabling free/busy times between organizations

You can achieve this scenario in 3 easy steps:

1. Create room in Organization A

Create a room in Organization A. For example,

2. Set up organizational relationship in Exchange Online for Organization A

  1. From the Microsoft 365 admin center dashboard, go to Admin > Exchange.
  2. Go to organization > sharing.
  3. Under Organization Sharing, click New +.
  4. In new organization relationship, in the Relationship name box, type a friendly name for the organization relationship.
  5. In the Domains to share with box, type the domain for the external Microsoft 365, Office 365, or Exchange on-premises organization you want to let see your calendars. If you need to enter more than one domain, separate the domain names with a comma. For example,
  6. Select the Enable calendar free/busy information sharing check box to turn on calendar sharing with the domains you listed. Set the sharing level for calendar free/busy information and set which users can share calendar free/busy information.
  7. Click save to create the organization relationship.

Both organizations should now be able to see each others free/busy times as well as available or occupied meeting room times via Outlook. Note: to see the meeting room in Organization A from Organization B, you will have to enter the meeting room email address (e.g.

The problem now is that if someone from Organization B tries to schedule the meeting room, the room will not process external request.

3. Allow external booking

In order for the meeting room to accept external request, just connect to Exchange Online PowerShell and run the following command:

Get-Mailbox "vitosha" | Set-CalendarProcessing -ProcessExternalMeetingMessages $true

This way has one disadvantage that is if Organization B wants to reserve room that belongs to Organization A, the domain of the room it must use belongs to Organization A. In some scenarios this is not good business practice and is seen by the meeting participants.

Sharing rooms by enabling forwarding

Using this method mitigates the need to use other organization domain in the meeting invite and employees can see rooms in both organization`s address book.

1. Creating rooms

Create rooms in both organizations. For example and

2. Do this in Organization A

Get-Mailbox "vitosha" | Set-CalendarProcessing -ProcessExternalMeetingMessages $true

Set-Mailbox -Identity "vitosha" -DeliverToMailboxAndForward $true -ForwardingSMTPAddress ""

3. Do this in Organization B

Get-Mailbox "vitosha" | Set-CalendarProcessing -ProcessExternalMeetingMessages $true

Set-Mailbox -Identity "vitosha" -DeliverToMailboxAndForward $true -ForwardingSMTPAddress ""

Now if user from Organization A wants to reserve room, he adds room to the meeting and room is automatically reserved also so both organization see the reserved slot. Information is also populated to both rooms of user updates or cancels meeting.

Mladen Georgiev
My name is Mladen Georgiev. My passion is new technology. I help companies in their efforts to transition to the Microsoft cloud. Security is one of my main focuses and I always strive to keep the balance between usability and protection. Always connected way of work presents new challenges which demand new modern approach to keep up with security threads. With this blog I want to share useful tips for making IT pro life easier and guides that help overcome real world challenges.

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