Share meeting rooms across Microsoft 365 or Google tenants

Why this is hard in Microsoft 365 (and Google)

A room mailbox lives in exactly one Microsoft 365 tenant. For people in that tenant booking is easy. Outlook shows the room in Room Finder, availability lookups work, and invitations get accepted automatically. For people in the other tenant, the room might as well not exist. Room Finder won't list it, availability lookups fail, and the only feedback is an accepted or declined email from a mailbox they can't see. Guest accounts don't fix scheduling, and cross-tenant sharing settings only soften the problem. Google Workspace has the same issue between organizations, and a mixed Google and Microsoft building has it twice.

The usual outcome is that the company that owns the room enjoys it, and everyone else books by emailing an office manager.

A neutral layer between the tenants

Naboro sits above the tenants instead of inside one of them. The shared calendar is the meeting point, and each organization connects its own calendar on its own side:

• Each org connects its own calendars. Company A syncs the rooms to calendars in its Microsoft 365 tenant, company B to its own tenant, or to Google or CalDAV. You don't need cross-tenant trust, guest accounts, or a federation project.
• People book where they already work. With two-way sync, a meeting created in your own Outlook or Google Calendar becomes a booking in Naboro, and bookings made in Naboro show up in your calendar. It takes seconds, not hours.
• Naboro settles conflicts. When two tenants' meetings collide, one wins and the other is declined in its home calendar, where the organizer can see it. There are no silent double bookings.
• Sync you can audit. Every sync run is logged, showing what was imported, mirrored, updated, or removed, and in which calendar. When IT asks what touched an event, there's an answer.
• Details stay inside each tenant. The other organization sees that a room is busy and who booked it, not your meeting's subject or attendees.

How to set it up

1. Create the building and rooms in Naboro. The shared calendar replaces the pretense that one tenant's room mailboxes work for everyone.
2. Invite each organization as a company. Each org's admin signs in with their own Microsoft or Google account.
3. Each org connects calendar sync. Per room, to calendars in its own tenant. Sync is priced per company, so each entity pays for its own side.
4. Remove the workarounds. You can finally delete the forwarding rules, the shared mailbox, and the rooms spreadsheet.