How to view customers invited to join but who have not registered

Click 'Customers' on the top left, then 'Main Dashboard' from the blue menu bar.  Scroll to the end of the Dashboards page and click 'Work with Not Registering' at the bottom of the 'Registration Breakdown' panel.

Here you can change the search criteria to show results from the last week, 1 month or 3 months. This will display customers that are not responding to notifications to view documents online.

From the search results you have the option to view the account or to send a reminder to the account.