Here is how to modify the query to do that:
List<Table> Source = new List<Table>();
var ANBA = from accountsTable in Source
orderby table.Extra
group table by (( table.Extra[0]>='a' &&table.Extra[0]<='z' )
||( table.Extra[0]>='A' &&table.Extra[0]<='Z' ) ?table.Extra.Substring(0,1).ToUpper()[0]:'*') into c
orderby c.Key
select new Group<Table>(c.Key.ToString(), c);
this.ANListGropus.ItemsSource = ANBA;