There is no mechanism in the UI to reply to all open tickets.
If you want a way to get the email addresses of all the users who currently have an open ticket you could do a SQL query like this:
SELECT `ost_user_email`.`address`
FROM `ost_ticket`
JOIN `ost_user` ON (`ost_ticket`.`user_id` = `ost_user`.`id`)
INNER JOIN `ost_user_email` ON (`ost_user`.`default_email_id` = `ost_user_email`.`id`)
WHERE `ost_ticket`.`status_id` = 1;
Which will give you a list of the email addresses for all users with open tickets.
