ST_Contains

select group_city, 
       ST_AsText(location) AS locationString,  
       ST_Contains(
         ST_GeomFromText('POLYGON ((
             -74.171737 40.607377, 
             -74.089339 40.753180, 
             -73.911498 40.769303, 
             -74.016555 40.604249,  
             -74.171737 40.607377))'),
	       toGeometry(location)
	     ) AS inPolygon
from meetupRsvp 
order by inPolygon DESC
limit 5

select count(*)
from meetupRsvp
WHERE ST_Contains(
         ST_GeomFromText('POLYGON ((
             -74.171737 40.607377, 
             -74.089339 40.753180, 
             -73.911498 40.769303, 
             -74.016555 40.604249,  
             -74.171737 40.607377))'),
	       toGeometry(location)
	     ) = 1