Hi partner,
It is possible to send automated mail to admin when a new user has been registered into portal.
Actually when a new user registered in portal, there will be a new contact record created in D365, so we only need create a Flow to create and send mail to administrator when a new contact has been created in D365 with owner is SYSTEM. (Due to the workflow in D365 could not choose SYSTEM as a condition in owner field, we could not choose workflow to do this)


Creating a new flow and set the trigger to when a record has been created.
Add a condition to check if the owner is SYSTEM.
If it is, then add a new action to send email, in Flow, you could send email by creating email record in D365 or send email by outlook directly.

Best Regards,
Leo