var query = from b in table.AsEnumerable()
                group b by b.Field<string>("поле1") into g                        
                let list = g.ToList()
                select new
                       {
                            key = g.Key,
                            count = list.Count(),
                            max = list.Max(x => x.Field<String>("цена")),
                       };

group b by
               b.Field<string>("поле1"),
               b.Field<string>("поле2")
into g    

let list = g.ToList()

group table by 

var query = from table in table1.AsEnumerable()
			group table by new { Col1 = table["поле1"], col2 = table["поле2"] } into groupby
			  select new
			  {
				  Value = groupby.Key,
				  max = groupby.Max(x => x.Field<String>("цена")),
				  count = groupby.Count(),
			  };