для получения информации о пользователях вы должны использовать соединение
$result = $this->db->query("
SELECT
`meetings`.*,
`follow_up`.id as follow_up_id,
`follow_up`.comment as follow_up_comment,
`follow_up`.date as follow_up_date,
`follow_up`.time as follow_up_time,
users.first_name,
users.last_name,
users.user_mobile,
users. useralt_mobile,
(SELECT address FROM day_location WHERE `meetings`.assigned_to_id = user_id AND `follow_up`.date = date LIMIT 1) AS location_name
FROM meetings
LEFT JOIN follow_up ON `meetings`.id = `follow_up`.`meeting_id`
LEFT JOIN users on users.id = `meetings`.assigned_to_id
WHERE follow_up.`date` BETWEEN '{$fromDate_formated}' AND '{$toDate_formated}'
" . ($user_id > 0 ? " AND `meetings`.assigned_to_id = '{$user_id}'" : '') . "
ORDER BY `follow_up`.id DESC");