Return sql results as a single string

Question

I have the following query which returns more than 1 result (max is about 10rows not more!), how can I return all results as a single string?

SELECT DISTINCT(e.Username)
FROM TblLeaveEmployee l
JOIN TblLeaveApprovalsBy a on l.EmployeeId = a.UserID
JOIN TblEmployee e on l.EmployeeId = e.id
where l.EmployeeId IN 
  (select UserID 
   FROM TblLeaveApprovalsBy 
   WHERE ApprovalUserID IN
     (SELECT ApprovalUserID from TblLeaveApprovalsBy where UserID = 77))

Have tried using STUFF as follows but its not working:

DECLARE @CodeNameString varchar(100)
SELECT 
   @CodeNameString = STUFF(
SELECT DISTINCT(e.Username)
FROM TblLeaveEmployee l
JOIN TblLeaveApprovalsBy a on l.EmployeeId = a.UserID
JOIN TblEmployee e on l.EmployeeId = e.id
where l.EmployeeId IN 
  (select UserID 
   FROM TblLeaveApprovalsBy
   WHERE ApprovalUserID IN
     (SELECT ApprovalUserID from TblLeaveApprovalsBy where UserID = 77)))

Answer 1

   DECLARE @CodeNameString VARCHAR(MAX)=NULL
   SELECT @CodeNameString = COALESCE(@CodeNameString+',' ,'') + Username 
   FROM
   (
   SELECT DISTINCT(e.Username) Username
   FROM TblLeaveEmployee l
   JOIN TblLeaveApprovalsBy a on l.EmployeeId = a.UserID
   JOIN TblEmployee e on l.EmployeeId = e.id
   where   l.EmployeeId IN (select UserID FROM TblLeaveApprovalsBy
              WHERE ApprovalUserID IN(SELECT ApprovalUserID from TblLeaveApprovalsBy    where UserID = 77))) x
   SELECT @CodeNameString