How to create a custom view (using lookup) from aggregated data
To disaggregate
SELECT id, 
   json_array_elements(array_to_json(string_to_array(csv_user_ids, ',')))
FROM groups


string_to_array converts the CSV string into ARRAY
array_to_json   converts ARRAY into JSON
json_array_elements each element into a separate row

next I just need to aggregate again.