How can we do a group reply to all the submissions in a survey?

Do you mean to say that you want to reply to all users who submitted your form?

Unfortunately we do not have any built in feature to achieve this. It is not possible to reply to all users and you need to reply to individual submissions from form's submission page.

Alternatively, you can export all your submissions, copy the email address of all users and then send an email to all of them from your own email client.