Thank you for posting this in Microsoft Q&A.I understand your ask whether if it is possible to use the External Identity User Flow to allow external users to self-sign up and be added to groups in Azure AD, and if so, how to achieve this.
Unfortunately, it is not possible to fulfill your requirement using the External Identity User Flow. User flow can be used to create sign-up and sign-in pages for your customers, but it does not have the ability to add users to groups.
For your reference: Add a sign-up-sign-in-customers in External tenants
You can find which groups and application roles are supported through Microsoft Graph and the admin center in the following document.
https://learn.microsoft.com/en-us/entra/external-id/customers/reference-group-app-roles-support
Hope this helps. Do let us know if you any further queries.
Thanks,
Navya.
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