First, you'll need to make sure that the Whiteboard app is enabled for your organization. To do this, go to the Microsoft Teams Admin Center and under Teams apps > Setup policies, make sure that the "Whiteboard" app is enabled.
Once you've confirmed that the app is enabled, you can then assign the app to the AD group you created. To do this, go to the Teams Admin Center and under Teams apps > Permission policies, create a new permission policy or edit an existing one. In the policy, add the Whiteboard app and then assign the policy to the AD group. After you've assigned the policy to the AD group, the users in that group should be able to access the Whiteboard app in Microsoft Teams.
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.