not sure about sql express 2008 but this is how it generally works:
First a join is created
from Employee left join [Achivements] on Employee.EmpID = [Achivements].EmpID
then groups are created:
group by FirstName, LastName
all rows with same firstname and lastname will fall in same group
the output will be one row for each group: firstname, lastname, and count of rows in the group
select FirstName, LastName, COUNT(CertificateId) as CertificatesAcheived