When dealing with Azure AD guest users, some errors can be tricky. Mix in some security requirements and it gets confused and forces requirements that can't be done.
Sometimes users change their mind and an office 365 group now needs to be a shared mailbox or they didn't like the email address they created in the first place. This post explains how to achieve this via Powershell.
How to update a PowerShell Window title and automate this with a PS profile
A modular script for connecting to Microsoft Online Services and on Premise infrastructure with MFA support.
Using PowerShell to filter events on the domain controller to find the admin account used to disable the user.