SQL Server 2012:Wybierz Top n na podstawie wielu kryteriów

Możesz to zrobić za pomocą row_number() :

select region, month, MemberId, sales
from (select region, month, MemberId, sum(sales) as sales
             row_number() over (partition by region, month order by sum(sales) desc) as seqnum
      from table t
      group by region, month, MemberId
     ) t
where seqnum <= 2;