cadena equivalente de Sum para concatenar

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

• Demostración de SQLFiddle

SALIDA

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