Hello @Edison Poon
Thank you for posting your query on Q&A.
To restrict the ability of users to register themselves for SaaS apps on the Microsoft Entrad ID platform and manage invitation and collaboration settings, you can proceed with the following steps:
Sign into the Microsoft Entra portal and navigate to the Microsoft Entra ID (AAD)
Browse to Identity > Users > User settings, and then under External users, select Manage external collaboration settings.
Set the Enable guest self-service sign up via user flows toggle to Yes and save.
I hope this answer helps! If you have any further questions, please feel free to ask.
For more information, please refer the below document.
https://learn.microsoft.com/en-us/entra/external-id/external-collaboration-settings-configure
https://learn.microsoft.com/en-us/entra/external-id/self-service-sign-up-user-flow
Thanks,
Akhilesh.
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