var query = (from c in context.customer_order
where c.id > 8000
group c.Column by c.person_id into cgroup
select new { cgroup.Key, Orders = cgroup });
Или, если вам нужно несколько столбцов s :
var query = (from c in context.customer_order
where c.id > 8000
group new { c.Column1, c.Column2 } by c.person_id into cgroup
select new { cgroup.Key, Orders = cgroup });