Thank you for posting in this community.Users in domain A cannot access files in domain B directly. For domain B, users in domain A are external users.
If you want users in domain A to be able to continue to access files in domain B, set the following settings in Domain B:
1.Configure external collaboration settings in Azure AD. External collaboration settings let you specify what roles in your organization can invite external users for B2B collaboration. If you don't want users in your domain A to be able to invite external users to domain B, then this setting can be skipped.
Here are the exact setup steps: Configure external collaboration settings.
2.Adding users in domain A as external users to Azure AD.
Here are the exact setup steps: Add guest users to the directory.
The user profile for users of Domain A is like DomainAemailaddress#EXT#@domainB.
3. Change the organization-level external sharing setting of sharepoint. Here set all guests who are already in your directory can access your SharePoint. You can choose the type of setting according to your situation.
Here are the exact setup steps: Change the organization-level external sharing setting.
4.Change the sharing settings for a site (where the files are stored).
Here are the exact setup steps: Change the sharing settings for a site. Here set all guests who are already in your directory can access a SharePoint site. You can choose the type of setting according to your situation.
5.Adding external users to site.
Go to the specific SharePoint site, click gear icon >> Site permissions >> Advanced permissions settings.
On the Site Permissions page, click on Grant Permissions to the external user.
If you have any questions in the setup, please feel free to let me know. We will do our best to help you.
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