I believe you'll want something like this:
$this->db->select('user_id, COUNT(user_id) as total');
$this->db->group_by('user_id');
$this->db->order_by('total', 'desc');
$this->db->get('tablename', 10);
This will produce a result like
| USER_ID | TOTAL |
| 12 | 3 |
| 15 | 2 |
| 18 | 1 |
UPDATE: As some pointed out in the comments the original query was summing the user_ids rather than counting them. I've updated the active record query to correct this.