Cannot add domain\User_Group to SQL Server Reporting Services Role Assignment (rsUnkownUserName)

I am trying to add a domain group similar to 'domain\Group_Name_Group' and regardless of the Home or folder level I get the Error:

The user or group name 'domain\Group_Name_Group' is not recognized.  (rsUnknownUserName).  I have searched and cannot find any group naming conventions for SQL Server Reporting Services.  I am running MS Server 2008 R2 Standard and SQL 2008 R2 Standard, both are 64-bit.

Thanks in advance,

Eric