If you can use the SQLCLR here is an Aggregate Function I wrote to simplify the syntax for these types of queries that performs comparably (sometimes better, sometimes slightly worse) than the XML method you were shown.

GROUP_CONCAT string aggregate for SQL Server[/url]

Here is what your query might look like:

SELECT POPRCTNM,

dbo.GROUP_CONCAT(CAST(ACTINDX AS VARCHAR)) AS AliasForStuff

FROM POP30390 d1

GROUP BY POPRCTNM;

If you wanted a duplicate-free list just add DISTINCT into the Aggregate:

SELECT POPRCTNM,

dbo.GROUP_CONCAT(DISTINCT CAST(ACTINDX AS VARCHAR)) AS AliasForStuff

FROM POP30390 d1

GROUP BY POPRCTNM;