With distinctsection As
(
SELECT DISTINCT section
FROM  dbo.TableA
)
SELECT section, names= STUFF((
    SELECT ', ' + b.Name
    FROM dbo.TableB as b
    JOIN dbo.TableA as a
    ON a.AccountId = b.AccountId
    WHERE a.section = ds.section
    FOR XML PATH, TYPE).value(N'.[1]', N'nvarchar(max)'), 1, 1, '')
FROM distinctsection ds