In Microsoft Office 365 Dedicated/ITAR (vNext), you can't add a security group or user to the permissions of a cross-forest room mailbox in Microsoft Outlook. Specifically, this issue occurs in the following scenario:
You were granted “Full Access” or folder level rights to a room mailbox in another forest.
You add the room mailbox as a shared Calendar in Outlook.
You right-click the Calendar folder and then select the Permissions tab:
You click Add, select a user or security group from the People Picker, and then click OK. The user or security group is displayed, and you can select a permission level :
When you click Apply or OK and then return to the Permissions tab, the user or security group is no longer listed:
Note This issue can be reproduced if the room mailbox is on-premises or in a legacy dedicated environment and the user is in vNext. This issue also occurs if the room mailbox was moved to vNext and the user has a mailbox on-premises or in legacy dedicated.
This issue is currently under investigation by Microsoft.
To work around this issue, use one of the following methods: