Until now, the whiteboard could not be used with guest accounts within Microsoft Teams. Which is a big limit as in many meetings guests can be invited. As Vesa Nopanen reports, Microsoft is now starting the rollout to support Guests Whiteboard Support in Teams.
It is also important to note that the function is not provided centrally for Tenants. Vesa Nopanen has written a nice blog about all the details of whiteboard with guests. Enjoy reading it.