We don't need to implode the array to use with WHERE.
$users = array(1,2,3);
$selectQry = $this->db->select()->from(array('FI'=>'TBL_NAME'))
->where('FI.user_id IN (?)',$users);
$users = array(1,2,3);
$selectQry = $this->db->select()->from(array('FI'=>'TBL_NAME'))
->where('FI.user_id IN (?)',$users);