This should work for you. Tested all the way back to SQL 2000.
create table #user (username varchar(25))
insert into #user (username) values ('Paul')
insert into #user (username) values ('John')
insert into #user (username) values ('Mary')
declare @tmp varchar(250)
SET @tmp = ''
select @tmp = @tmp + username + ', ' from #user
select SUBSTRING(@tmp, 0, LEN(@tmp))