Customers can create an account but can not login later

It sounds like you used formidable form to place an Email Address form within your website. You can set the form up to send a notification to the user once they enter their email address. But it looks like if you wanted to Register the user that just submitted the email address, that is a pro feature. If you are a pro feature user, here is a screen shot to show you where to click.

HERE

Assuming I answered actually what you were asking, I hope this helps.