Come concatenare molte righe con lo stesso ID in sql?

In SQL Server puoi farlo nel modo seguente:

DOMANDA

SELECT id, displayname = 
    STUFF((SELECT DISTINCT ', ' + displayname
           FROM #t b 
           WHERE b.id = a.id 
          FOR XML PATH('')), 1, 2, '')
FROM #t a
GROUP BY id

DATI DI PROVA

create table #t 
(
id int,
displayname nvarchar(max)
)

insert into #t values    
 (1 ,'Editor')
,(1 ,'Reviewer')
,(7 ,'EIC')
,(7 ,'Editor')
,(7 ,'Reviewer')
,(7 ,'Editor')
,(19,'EIC')
,(19,'Editor')
,(19,'Reviewer')

USCITA

id  displayname
1   Editor, Reviewer
7   Editor, EIC, Reviewer
19  Editor, EIC, Reviewer