Hi @NS Admin •
Thank you for Reaching out Microsoft Q&A forum.
I understand that you are attempting to add a new user who has already created an account using your domain email address. However, when you try to invite them as an external user, you are encountering issues, as the system does not allow you to invite the external user.
Based on your query, the user is created using your domain email address, meaning they are using an existing email address within Azure. It automatically recognizes as an existing domain email address, so it won't allow you to invite the external user. You shouldn’t need to invite them as an "external user" as they are already part of your domain or tenant.
Hope this helps. Do let us know if you any further queries.
Best Regards
Harshitha Eligeti.