update person/group field in powershell, with a group name

Hi,

I need to update a person column in my library through powershell, I can do this for a single user, but how do update the value to be a sharepoint group?  the column has been set correctly to allow groups, and I can set the group name if I use the browser. but I cant do it through powershell. I get "Invalid data has been used to update the list item"

Thanks,