From the homepage select Manage for an issue followed by the Analytics button. You can also select Analytics from the main menu and then navigate to an issue.
A 'visits' percentage for each expected visitor list will display on the reporting dashboard for the issue and can be selected for a breakdown of data.
Once selected you will be presented with a pie chart of data and two tables containing the email address of the people that have 'visited' and 'not visited'. Both lists can be downloaded.
Why not download the spreadsheet of 'not visited' email addresses and use it as a contact list to send out an email reminder?