Postgresql Total elements and affiliation users
You need to remove user_id from your group by clause. But that requires putting it into an aggregate function in the SELECT list. One way to do that is to use string_agg()
SELECT its.item_id,
       its.tag,
       count(its.tag) as tags_count,
       string_agg(u.user_id, ',') as user_ids
FROM items_tags_sets its
  LEFT JOIN users u on u.user_id = its.user_id and u.user_id = 'usr1'
WHERE its.item_id = 'it1'
GROUP BY its.item_id, its.tag
order by its.item_id, its.tag;

Online example