Linq group by multiple fields across different tables

In an attempt to convert this SQL query to LINQ I have hit a problem with group by. How can I group by multiple columns that originate from multiple tables? The linq query below simply won't compile so I'm in need of the correct syntax

Original SQL Query

select LTRIM(RTRIM(e.Shortname)) 
Name, LTRIM(RTRIM(j.JobName)) 
JobName, j.JobCode, 
SUM(Hours) Hours, 
MAX(ce.LatestTimesheetEntry) LastTimeSubmitted 
from TWCTL.TW.Postings p
join TWCTL.TW.Employees e on e.EmployeeId = p.EmployeeId
join TWCTL.TW.Jobs j on j.JobCode = p.JobCode
join CentralTimeEmployees.dbo.Employees ce on ce.CtEmployeeId = e.EmployeeId
where (e.CostCentreId = 1 or e.CostCentreId = 3) 
and (p.TransactionDate >= '2012-01-01' and p.TransactionDate <= '2012-07-01') 
and j.JobCode <> 'CTCIT00001' 
and ce.DatabaseCode = 'CTL' 
and (ce.CostCentreId = 1 or ce.CostCentreId = 3)
group by j.JobName, j.JobCode, Shortname
order by e.Shortname, j.JobName

Linq Query attempt (not working)

var model = 
        from p in context.Postings
        join e in context.Employees on p.EmployeeId equals e.EmployeeId
        join j in context.Jobs on p.JobCode equals j.JobCode
        join ce in context.CentralTimeEmployees on e.EmployeeId equals ce.CtEmployeeId
        where (e.CostCentreId == 5 || e.CostCentreId == 3)
                && (p.TransactionDate >= fromDate
                    && p.TransactionDate <= toDate)
                && j.JobCode != "CTCIT00001"
                && ce.DatabaseCode == "CTL"
                && (ce.CostCentreId == 1 || ce.CostCentreId == 3)
        group j by new {j.JobName, j.JobCode} into g1
        group e by e.Shortname into g2    <- doesnt work??        
        select
            new ProjectHoursViewModel
                {Posting = p, Job = g1.Key.JobName, Employee = e, CentralTimeEmployee = ce};