Cross-tenant calendar sharing

For organisations doing significant cross-tenant collaboration — joint ventures, parent-subsidiary relationships, ongoing partner work — the ability to see free/busy across tenants when scheduling meetings is operationally valuable. Microsoft 365 supports this via organisation relationships in Exchange Online, with several flavours.

Exchange organisation relationships

The classic mechanism: configure an organisation relationship between two tenants. Once set up, users in Tenant A can see free/busy of users in Tenant B (with optional level — busy only, limited details, full details).

Configuration is bidirectional — each side configures the inbound relationship from the other. Set up via Exchange Online PowerShell with New-OrganizationRelationship:

# On Tenant A
New-OrganizationRelationship -Name "TenantB" `
    -DomainNames "tenantb.com" `
    -FreeBusyAccessEnabled $true `
    -FreeBusyAccessLevel AvailabilityOnly

Plus matching on Tenant B. Within hours, users on each side see free/busy of the other.

Limits of the classic model

Organisation relationships have caveats:

• Free/busy only — calendars themselves aren't shared, only availability.
• Doesn't extend to Teams automatically — Teams scheduling assistant uses Exchange data so generally works, but some scenarios are limited.
• DNS configuration required for federation discovery to work.
• Trust is by domain — anyone signing up tenantb.com could theoretically configure their tenant against yours; rare but possible.

Modern alternative: Multi-Tenant Organisations

For organisations operating multiple Microsoft 365 tenants as one logical entity, Entra ID Multi-Tenant Organizations (MTO) provides a richer model:

• Unified people search across MTO tenants.
• Cross-tenant user synchronization auto-creates guests in each tenant.
• Shared channels in Teams between MTO members.
• Cross-tenant Microsoft Search finds content across tenants.

MTO is the right answer for parent / subsidiary / acquisition scenarios where the tenants belong to the same overall organisation. Free/busy still requires organisation relationships, but the broader collaboration story is much richer.

B2B guest pattern

For ongoing partner collaboration, B2B guests in each other's tenants give the cleanest user experience:

• The user is a member of Tenant A (their employer) and a guest in Tenant B (the partner).
• They see Tenant B's calendars naturally when logged into Tenant B as the guest.
• No federation configuration needed.

Best for named individuals doing deep collaboration, less practical for entire workforces seeing each other's free/busy at scale.

Microsoft Bookings for external scheduling

For external customer-facing scheduling, Microsoft Bookings lets external customers book time on a staff member's calendar without any tenant relationship. The customer doesn't see free/busy directly — they see available booking slots. Works for client scheduling, partner-to-partner appointments, external advisor meetings.

Operational considerations

• Document the relationship so future admins know it exists and why.
• DNS for federation discovery — must be configured correctly.
• Periodic review — old organisation relationships from former partners should be removed.
• Combine with Cross-Tenant Access Settings for the B2B trust layer.

For organisations with significant ongoing cross-tenant work, the right answer depends on the relationship's depth:

• One-off external attendees — Microsoft Bookings or anonymous meeting invites.
• Specific partner individuals — B2B guests.
• Whole-organisation visibility — Exchange organisation relationships.
• Sister tenants of the same parent — Multi-Tenant Organization.

Pick the right tool for the relationship; configure once, benefit forever.