SELECT
    GroupColumn,
    STRING_AGG(TargetColumn, ',') WITHIN GROUP (ORDER BY TargetColumn) AS MergedStrings
FROM
    YourTable
GROUP BY
    GroupColumn;

SELECT
    GroupColumn,
    STUFF(
        (SELECT ', ' + TargetColumn
         FROM YourTable t2
         WHERE t2.GroupColumn = t1.GroupColumn
         ORDER BY TargetColumn
         FOR XML PATH(''), TYPE).value('.', 'NVARCHAR(MAX)'), 
    1, 2, '') AS MergedStrings
FROM
    YourTable t1
GROUP BY
    GroupColumn;