How to Reply to the Users Manually?

While I have added the autoresponder, is there any way to go back to the 7 registrants who have already registered to send them the autoresponder?

Creating "Autoresponder" isn't send confirmation email to them; but you can achieve this scenario on "Submissions" by sending email manually. 

To do that,

1) Open "Submissions".

2) Then, select the desired submission and click to the "Reply" button. On the appeared popup window, you can edit and send the email to that user.

If you encounter any error or need extra information, please do let us know.

Thanks.