If your website has an email sign up form you can use Google Analytics to keep track of which email addresses are signing up. Simply use the parameter you are using to pass the email address as the query parameter in your Analytics Site search setup (go to Profile Settings and click Edit Profile on the profile you want to add this to).
Use parameter you pass on the email address sign up as your Site Search parameter
Of course, you’ll probably want to use a duplicate profile for this to avoid skewing the data on your main profile. If you don’t know how to create a duplicate profile, I explain how on Track outbound links and file downloads with Google Analytics.
You can now track email address sign ups in Google Analytics.
The usefulness of this technique doesn’t end here. The Site Search section allows you to view performance of visitors who used your site’s keyword search. In this case, you can use the same tools to see how visitors using the email sign up form are performing on your website. You can also use Advanced Segments to compare how people who use the email sign up form behave (pages/visit, bounce rates, conversion rates, goal completions, etc) compared to those who don’t. Learn how create Advanced Segments here if you don’t know how to use them.