Try this code.
declare @teamLead as int
declare @eid int
select @eid = 4
select @teamLead=TM_UserID from TM_USER where Report_To=@eid
if @teamLead is null
begin
select @teamLead=Report_To from TM_USER where TM_UserID=@eid
end
else
begin
select @teamLead=@eid
end
(select distinct emp.TM_UserID,emp.FullName,TL.FullName as TeamLead from TM_User EMP
join TM_User TL on emp.Report_To=tl.TM_UserID where ( emp.Report_To=@teamLead ))