How to assign AD group default schema

Hi,

We have AD group. Users are member of this group. I want to assing default_schema to this AD group. Why I right click user in db, selected properties, default schema is not selectable. How to to assing default schema to a windows AD group? (Microsoft SQL Server 2008 R2 (SP2) - 10.50.4000.0 (X64).

Thanks