This is how to work around the Microsoft Teams issue: “Video sharing is disabled by the administrator”.

This occurs when the meeting policy applying to the organizer has:
To reproduce: Invite an external user, they join before the organizer, and then get the message “Video sharing is disabled by the administrator” with the camera icon greyed out. If the organizer leaves and rejoins, video starts working.
We raised this with Microsoft and were told it’s by design — because the policies cannot apply before the organizer starts the meeting, Teams doesn’t know if video should be disabled or not.