Currently, RoomMailbox permissions are managed via the ‘Exchange Advanced’ tab (‘Mailbox Roiights’) of the object’s properties.
But, you may have noticed that while managing a ‘ShareMailbox’ object, the product offers a management tab named: ‘Mailbox Sharing’ to expedite the permissions assignment task.
An enhancement request (TF00781687) has been created detailing the feature: Display 'Mailbox Sharing' tab for the ‘RoomMailbox’ properties as displayed for the ‘SharedMailbox’ properties.
The product team will evaluate the request and this feature may become available on a future release of the product.
Please refer to this article for updates or contact support referencing the Enhancement Request ID: TF00781687.