if you're doing a report that is grouped by user ID, use the method that Sastry suggested...by far the best if you're grouping by user ID...you won't need to do a subreport in that case or a command or sql expression etc.
however, if you're grouping by something that is not user ID, see the methods i mentioned before.
e.g. if your report is grouped by Month, then you can put in a cross-tab at the month level and that cross-tab has the user id as the row, and the count of user id as the summary. this gives you the user id counts by month.
or if you're using a command object (this is not necessary unless you're getting into a more complex report) then you can write something like the following where your prompts / parameters for the date range are in the command itself.
SELECT
`Orders`.`Order ID`,
`Orders`.`Employee ID`,
(
SELECT COUNT(`Orders2`.`Employee ID`)
FROM Orders Orders2
WHERE Orders2.`Employee ID` = Orders.`Employee ID`
) AS COUNT
FROM `Orders` `Orders`
WHERE `Orders`.`Order Date` >= {?DateStart}
AND `Orders`.`Order Date` <= {?DateEnd}