select
   distinct  
    stuff((
        select ',' + u.username
        from users u
        where u.username = username
        order by u.username
        for xml path('')
    ),1,2,'') as userlist
from users
group by username

had a typo before, the above works

 select
   distinct  
    stuff((
        select ',' + u.username
        from users u
        where u.username = username
        order by u.username
        for xml path('')
    ),1,1,'') as userlist
from users
group by username

had a typo before, the above works

 select
   distinct  
    stuff((
        select ',' + u.username
        from users u
        where u.username = username
        order by u.username
        for xml path('')
    ),1,1,'') as userlist
from users
group by username

had a typo before, the above works

 select
   distinct  
    stuff((
        select ',' + u.username
        from users u
        where u.username = username
        order by u.username
        for xml path('')
    ),1,2,'') as userlist
from users
group by username

had a typo before, the above works