Introduction

In today’s interconnected business world, collaboration across organizations is essential. Microsoft 365 offers robust tools for cross-tenant calendar sharing, allowing users from different tenants to view availability (free/busy) or even full calendar entries. This guide walks you through the setup process, drawing from official Microsoft documentation and practical examples. We’ll cover free/busy sharing via organization relationships and full calendar access, which requires enabling external sharing and per-user configurations.

Possible Scenarios for Cross-Tenant Calendar Sharing

Cross-tenant sharing is useful in various contexts. Here are some common scenarios:

  • Business Partnerships: Two companies collaborating on a project need to schedule meetings without conflicts. Free/busy sharing ensures visibility into availability, while full sharing allows detailed event insights for closer coordination.
  • Mergers and Acquisitions: During integration, teams from acquired and parent companies can share calendars to align on timelines, reducing miscommunication.
  • Supply Chain Management: Vendors and clients can view each other’s schedules for deliveries or reviews, enhancing efficiency.
  • Hybrid Work Environments: Freelancers or consultants from external tenants can integrate calendars for seamless booking.
  • Multi-Tenant Organizations (MTO): In advanced setups like MTO, sharing extends to groups, but starts with basic free/busy for all users.
  • Challenges in Regulated Industries: Sectors like finance or healthcare may limit to free/busy only for compliance, avoiding full entry details.

Note: Full sharing beyond free/busy isn’t automated tenant-wide; it’s user-initiated. For large-scale needs, consider automation scripts or policies.

Prerequisites

  • Global or Exchange Admin permissions in both tenants.
  • Access to Microsoft 365 Admin Center and Exchange Admin Center.
  • Exchange Online PowerShell module for advanced setups.
  • Domain names of both tenants (e.g., contoso.com and fabrikam.com).
  • Changes may take up to 24 hours to propagate.

Step-by-Step Setup

Step 1: Enable External Calendar Sharing (Required for Full Access)

This allows users to share calendars externally.

  1. Log into the Microsoft 365 Admin Center (admin.microsoft.com).
  2. Go to Settings > Org settings > Services > Calendar.
  3. Enable Let your users share their calendars with people outside of your organization who have Office 365 or Exchange.
  4. Select access levels (e.g., full details).
  5. Save and repeat in the other tenant.

Step 2: Set Up Organization Relationship (for Free/Busy Sharing)

This enables tenant-wide visibility of availability. Perform in both tenants for bidirectional access.

Using Exchange Admin Center

  1. Log into Exchange Admin Center (admin.exchange.microsoft.com).
  2. Navigate to Organization > Sharing > Organization sharing tab.
  3. Click + Add organization relationship.
  4. Enter a name (e.g., “Contoso-Fabrikam”).
  5. Add the other tenant’s domain.
  6. Enable Calendar free/busy information sharing.
  7. Choose level: Time only or Time, subject, location.
  8. Select scope: Everyone or a security group.
  9. Create and repeat in the other tenant.

Using PowerShell

  1. Connect: 
    Connect-ExchangeOnline -UserPrincipalName [email protected]
  2. Create: 
    New-OrganizationRelationship -Name "Contoso-Fabrikam" -DomainNames "fabrikam.com" -FreeBusyAccessEnabled $true -FreeBusyAccessLevel LimitedDetails
  3. Verify: 
    Get-OrganizationRelationship | Format-List

Step 3: Perform Full Calendar Sharing (Per-User)

Once external sharing is enabled:

  1. In Outlook, right-click your calendar and select Share > Permissions.
  2. Add the external user’s email.
  3. Set permissions (e.g., Can view all details).
  4. Send invitation; recipient accepts in Outlook.
  5. To view: Add the shared calendar via link or invitation.

Verification and Troubleshooting

Test by inviting cross-tenant users to meetings. If issues arise, check domains, wait for propagation, or use PowerShell diagnostics. For advanced scenarios like MTO, refer to Microsoft docs.

Conclusion

Cross-tenant calendar sharing boosts productivity but requires careful setup for security. Start with free/busy for broad access, then enable full sharing as needed. Stay updated with Microsoft changes!

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