MS SQL JOIN problem

I bet this is dead easy for someone who knows SQL.  

I am trying to pull back some data from a couple of databases.  I'm not fluent with joins yet, so I create a view to get what I want, then copy the SQL code to use it in a SP.  

I have created the following View in the StockTake database:

SELECT        dbo.StockItems.ID, dbo.StockItems.Status, dbo.StockItems.Warehouse, dbo.StockItems.StockCode, SysproCompanyD.dbo.InvMaster.Description AS StockDesc, SysproCompanyD.dbo.InvWarehouse.UnitCost, 
                         SysproCompanyD.dbo.InvWarehouse.QtyOnHand AS SystemQuantity, SUM(dbo.StockEntries.SlipQuantity) AS Total, SUM(dbo.StockEntries.SlipQuantity) 
                         - SysproCompanyD.dbo.InvWarehouse.QtyOnHand AS Variance, CAST(SUM(dbo.StockEntries.SlipQuantity - SysproCompanyD.dbo.InvWarehouse.QtyOnHand) 
                         * SysproCompanyD.dbo.InvWarehouse.UnitCost AS numeric(18, 2)) AS VarianceValue
FROM            dbo.StockItems INNER JOIN
                         dbo.StockEntries ON dbo.StockItems.StockCode = dbo.StockEntries.StockCode INNER JOIN
                         SysproCompanyD.dbo.InvMaster ON dbo.StockItems.StockCode = SysproCompanyD.dbo.InvMaster.StockCode INNER JOIN
                         SysproCompanyD.dbo.InvWarehouse ON SysproCompanyD.dbo.InvMaster.StockCode = SysproCompanyD.dbo.InvWarehouse.StockCode
GROUP BY dbo.StockItems.Warehouse, dbo.StockItems.Status, dbo.StockItems.StockCode, SysproCompanyD.dbo.InvMaster.Description, SysproCompanyD.dbo.InvWarehouse.UnitCost, 
                         SysproCompanyD.dbo.InvWarehouse.QtyOnHand, dbo.StockItems.ID

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It pulls entries from the stocktake application I wrote and joins it to a syspro database to pull back certain fields related to the stocktake.  This all works

I have now been asked to show only the stock items in the syspro database that have NOT shown up in the stocktake.  To do this, I removed the StockEntries table as it is now not needed.  Then I tried changing to a right join instead of the inner join as below:

SELECT        dbo.StockItems.ID, dbo.StockItems.Status, dbo.StockItems.Warehouse, dbo.StockItems.StockCode, SysproCompanyD.dbo.InvMaster.Description AS StockDesc, SysproCompanyD.dbo.InvWarehouse.UnitCost, 
                         SysproCompanyD.dbo.InvWarehouse.QtyOnHand AS SystemQuantity
FROM            dbo.StockItems INNER JOIN
                         SysproCompanyD.dbo.InvMaster ON dbo.StockItems.StockCode = SysproCompanyD.dbo.InvMaster.StockCode RIGHT OUTER JOIN
                         SysproCompanyD.dbo.InvWarehouse ON SysproCompanyD.dbo.InvMaster.StockCode = SysproCompanyD.dbo.InvWarehouse.StockCode
GROUP BY dbo.StockItems.Warehouse, dbo.StockItems.Status, dbo.StockItems.StockCode, SysproCompanyD.dbo.InvMaster.Description, SysproCompanyD.dbo.InvWarehouse.UnitCost, 
                         SysproCompanyD.dbo.InvWarehouse.QtyOnHand, dbo.StockItems.ID
where StockCode is null

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I get lots of rows when I don't have the Where clause.  When I include the where clause, I get the error below.  

Error in GROUP BY clause.
Unable to parse query text.

I ok that and up pops a message saying

Incorrect syntax near the keyword WHERE

I get this error and I'm stumped.  I tried adding a Group By clause and it made no difference.  

To be brief in case I waffled a bit, I want all the records from the syspro database where the stockcode is not included in the stockTake database.

Thanks in advance

John