try this,
;WITH temp as
(
SELECT DISTINCT
u.TM_UserID
task_name
FROM
project
JOIN task
ON project.project_id=task.project_id
JOIN timesheet
ON timesheet.task_id=task.task_id
JOIN Team
ON project.project_id = Team.project_id
JOIN TM_user u
ON u.TM_userID=timesheet.user_id
WHERE u.Is_Active=1
AND u.report_to=13
AND worked_dte BETWEEN '2014-03-18' AND '2014-03-21'
)
select
task_name
,STUFF(
(
SELECT ', ' + U1.FullName
from TM_user
INNER JOIN temp ON temp.task_name = temptbl.task_name
FOR XML PATH ('')
),
1,
2,
''
) FullName
from temp temptbl
Group BY task_name
Happy Coding!
:)