Ok, you'll want to derive the week based on the daten and group on the derived week field. The easiest way to do this would probably be to create a lookup table for the weeks of the year. It would include the beginning and end date for each week as well as the week number.
After doing that, try using something similar to the following query:
select week, employeename, sum(hours) from mytable inner join MyWeeks on MyWeeks.BeginDate<=mytable.WorkDate and MyWeeks.EndDate>=mytable.WorkDate Group By employeename, week
Dave Robinder, MCSD