On my form I believe that I have properly set it up to add a sequential number on each submission

I would like the autorespond email to also contain that number.

How can I create a registration form that auto generates a sequential membership number that is then passed on to the autorespond email?

Hi,

Thank you for contacting us.

You may use the Unique ID Widget, it will generate a unique value for each submission, then it will be added to your email template.

You may also check this guide for further info : http://www.jotform.com/help/79-Add-Custom-Unique-IDs-to-your-Form-Submissions 

Also, if the value is not automatically added to your email notification, you may add it by opening the email and then adding it, you should find it on the right side.

Hope this will help you.

Feel free to contact us if you need more help. 

Thanks.