SELECT  
    DATEADD(DAY, 7 - DATEPART(WEEKDAY, orderDate), CAST(orderDate AS DATE)) AS ordersPerWeek, 
    sum(orderAmount) AS orderSumForGrouping, 
    count(1) AS numberOfOrdersWithinGrouping
FROM    @table o
GROUP   BY DATEADD(DAY, 7 - DATEPART(WEEKDAY, orderDate), CAST(orderDate AS DATE))