string equivalent of Sum to concatenate

SELECT
     p.ID PersonID,
     STUFF(
         (SELECT ';' + b.description
          FROM  personrole a 
                INNER JOIN role b
                  ON a.roleid = b.id
          WHERE a.personid = p.id
          FOR XML PATH (''))
          , 1, 1, '')  AS DescriptionList
FROM person AS p
GROUP BY p.ID

• SQLFiddle Demo

OUTPUT

╔══════════╦════════════════════════════╗
║ PERSONID ║      DESCRIPTIONLIST       ║
╠══════════╬════════════════════════════╣
║        1 ║ user                       ║
║        2 ║ user;admininstrator;tester ║
╚══════════╩════════════════════════════╝

SQL Server (starting with 2017) supports OOTB STRING_AGG function

select p.id, STRING_AGG(pr.description, ',') as roles 
from person p 
inner join personrole pr ON p.id = pr.personid
inner join roles r ON r.id = pr.roleid
group by p.id