I guess I got lucky with my google foo.
I knew it could be done – initially thought it was Office 365 Groups then realized that wasn't the right one so changed my search criteria.
Anyhow, I'm sure you'll be able to follow the instructions.
The other issue you have is every damn Global Admin also becomes a user in D365 – albeit with no permissions so watch that as well.
In my opinion the 3rd party should have their own security role (it doesn't even have to do anything – could even be empty) and just count users assigned that role but I guess that would require someone to think Save a lot of stuffing around.
If you've found this thread useful, dive deeper into User Group community content by role