WITH pt AS (
SELECT 1 as id, 'point(-75.5 47.2)'::geometry geom
  union
SELECT 2 as id, 'point(-75.6 47.3)'::geometry geom
),
poly AS (SELECT 1 as id, 'polygon((-70 51, -81 51, -81 40,-70 40, -70 51))'::geometry geom)
SELECT poly.id polyID,  string_agg(pt.id::text,',') ptIDs
FROM pt, poly
WHERE st_intersects(poly.geom, pt.geom)
GROUP BY poly.id;

 polyid | ptids
--------+-------
      1 | 1,2
(1 row)