Thank you for reaching out to Microsoft!
Unfortunately, the Grant-CsApplicationAccessPolicy cmdlet does not support directly assigning policies to groups. It is designed to assign policies to individual users only.
Graph API can work either on delegated permission or Application-level permissions. However, you can limit application permissions to a specific group by creating a New-ApplicationAccessPolicy. This method should satisfy your requirement.
For more information, please refer to the following documentation: Limiting application permissions to specific Exchange Online mailboxes
Hope this helps.
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