For a long time, the methods of having two Azure AD tenants aware of each other’s users needed to be managed in either a manual, or scripted way; accessing the data of another tenant or using their configured Apps would require each user to enrol to the other tenant and be given default guest permissions; or an admin at the destination tenant would need to set things up, send invites out, or do something else creative to make the user experience better.
I was on board Azure AD B2B in the early days; as a Microsoft MVP I had the privilege of speaking to a product manager for it that one time I went to Redmond, talking about my use case and seeing if I was ‘doing it right’. A combination of Azure AD B2B and Azure App Proxy I’d set up for guest accounts to get into an internally hosted web based application, and it worked quite well. I had my own script going through a many step process to send out an invite to the user, add the user to multiple groups and whatever other trickery I needed at the time.
Cross-tenant synchronization however, takes a lot of that pain away. You can set up a trust between two Azure AD tenants (which can be a one way sync) to allow users in Tenant A to be automatically created and managed in Tenant B as a guest user. This is great for organisations who have to frequently work with another org – and even though it’s early days for cross-tenant sync, there’s some rather good controls already. You aren’t limited to a single relationship either; I can’t see any documented limits.
Attribute Mapping allows you to configure extra rules around the attributes that get passed on, allowing you to manipulate, add or remove certain attributes (you might want to remove an employee number from employeeid, or add an extra attribute to define what tenant they were synced from; or do something that will in turn match a dynamic security group rule to automatically add your synced users to be allowed to access an application.
I’d often step through how to set this up in one of these articles, but the documentation is already detailed with step-by-step screenshots and clear instructions. It worked exactly as described when I set this up between two test tenants I have, and took about 15 minutes beginning to end, which included reading the documentation a few times to make sure I was following it correctly. It’s also possible to do via Graph API, but I did not try this method.
There’s even detailed sync logs, troubleshooting tips, and detailed reporting.
One question I’ve seen multiple people already ask is how does this relate to the Global Address List (GAL) and People Search – which the documentation claims this isn’t on by default, but easy to enable. In my testing however, the accounts showed up in the GAL with the little ‘blue person in front of world’ symbol with no extra configuration. They didn’t turn up instantly and I waited overnight, then they were there. People Search was the same. If you want to investigate this for yourself, check out the showInAddressList attribute. Other documentation also says guest objects aren’t in the GAL by default too:
As always, be aware that this is Public Preview so has less guarantees than a fully launched feature. If you have any feedback or want to see what others might be saying/asking, check out the official feedback for Azure Active Directory.
Worth mentioning licensing.
As per What is a cross-tenant synchronization in Azure Active Directory? (preview) – Microsoft Entra | Microsoft Learn:
In the source tenant: Using this feature requires Azure AD Premium P1 licenses. Each user who is synchronized with cross-tenant synchronization must have a P1 license in their home/source tenant. To find the right license for your requirements, see Compare generally available features of Azure AD.
In the target tenant: Cross-tenant sync relies on the Azure AD External Identities billing model. To understand the external identities licensing model, see MAU billing model for Azure AD External Identities
The MAU billing section:
In your Azure AD tenant, guest user collaboration usage is billed based on the count of unique guest users with authentication activity within a calendar month. This model replaces the 1:5 ratio billing model, which allowed up to five guest users for each Azure AD Premium license in your tenant. When your tenant is linked to a subscription and you use External Identities features to collaborate with guest users, you’ll be automatically billed using the MAU-based billing model.
Your first 50,000 MAUs per month are free for both Premium P1 and Premium P2 features. To determine the total number of MAUs, we combine MAUs from all your tenants (both Azure AD and Azure AD B2C) that are linked to the same subscription.
The pricing tier that applies to your guest users is based on the highest pricing tier assigned to your Azure AD tenant. For more information, see Azure Active Directory External Identities Pricing.
Then from Pricing – Active Directory External Identities | Microsoft Azure:
Each synced user needs an Azure AD Premium P1 or P2 license in their home tenant.
Each tenant receiving synced users has the Azure AD External Identities billing model which used to be a 1:5 model, but is now 50k users free, the rest a small charge per active user.
Does a synced account count as an active user? Unsure, I would guess it’s a ‘probably not’ since there’s no active login for just existing as a guest in another tenant, but verify that for yourself with your licensing reseller.