Powershell Question--Add security group to Room Resources

I'm running the following script...

foreach($room in import-csv rooms.csv){
$BIP = (Get-CalendarProcessing $room.Name).Bookinpolicy
$BIP += UserName
Set-CalendarProcessing $room.Name -Bookinpolicy $BIP
}

 
However, when I use a security group name in the "UserName" section I get an error 'User does not exist'. I simply want to add security group to the BookInPolicy section of a list of Rooms instead of list of users.


Note: I created a DG and used -type Security. The security group works in other areas.