you'd also have to OUTER JOIN any tables linked off of Orders as well, so maybe:

--first half of union
FROM dbo.Account AS ACC
   INNER JOIN dbo.Groups AS GRO ON ACC.GROUPID = GRO.GROUPID
   LEFT OUTER JOIN dbo.Orders AS ORD ACC.ACCOUNTID = ON ORD.ACCOUNTID
   LEFT OUTER JOIN dbo.Products AS PRO ON PRO.PRODUCTID = ORD.PRODUCTID
   LEFT OUTER JOIN dbo.Sales AS SAL ON ACC.SALESID = SAL.SALESID

--second half of union
FROM Account ACC
   INNER JOIN Groups GRO ON ACC.GroupID = GRO.GroupID
   LEFT OUTER JOIN Orders ORD ON ACC.AccountID = ORD.AccountID
   LEFT OUTER JOIN OrderDetail ORDD ON ORD.OrderID = ORDD.OrderID
   LEFT OUTER JOIN Products PRO ON PRO.ProductID = ORDD.ProductID
   LEFT OUTER JOIN dbo.Sales AS SAL ON ACC.SALESID = SAL.SALESID