Firstly I must ask you, how actionable do you think it is to look at data at that granular of a level? Finding out what % of people who registered came from facebook or some other place is actionable, because it helps you do things like determine where to focus marketing efforts. But individual users? How is this actionable to you? (hint: it's not)
However, if you are still determined to know this, you should first note that it is against Google's ToS to record personally identifiable data both directly (recording the actual value in GA) or indirectly (e.g. - recording a unique id that you can use to tie to personal info stored within your own system). If this is something you don't want to risk, I suggest moving to another analytics tool that does not have this sort of thing in their ToS (e.g. Adobe SiteCatalyst, which costs money, or perhaps you may instead prefer to choose an "in-house" approach, like Piwik)
If you are still determined to follow through with this and hope not to get caught or whatever, Google Analytics doesn't record data like what info a visitor filled out in a form (like their email address) unless you populate that data in a custom field/dimension/metric/event to be sent along with the request. Usually you would populate this on the form "thank you" page (which is usually the same page you use as your goal url or goal event if you're popping and using an event for your goal). So you would populate the email address in one of those custom variables and then have it as a dimension to break down the http referrer by.