Capture the users data even if a form is not submitted

Also, how can I access this data even if the registrant does not click submit?

Unfortunately, no. You can only have access to data a user entered in a form when the form is submitted. 

Can you propose this feature be added? A great way to keep leads..
...

I will raise your request to our developers. However, please note that we cannot provide you the exact time when and if this will be implemented. If there is any update we will inform you via this thread. 

For now, I can only suggest you to use two forms. The first form can have only an email field. When the first form is submitted you can redirect your users to the second form (main form).

Redirecting Users to a Different Page

Then you will be able to compare the email addresses on the submission page of both forms to see if the users submitted the second form or not. You will also capture an email address of the users, and you will be able to contact them.