This depends on you having up-to-date attendee data loaded in the event already.

To send email to members who have not yet registered for an event:

1. Go to the Member Manager
   
2. Open the Advanced Filtering dialog
   
3. At the bottom, in the Additional Data to Include section, choose the event from the dropdown on the Event line.
   
4. Back at the top, in the Custom Filter editor, add a new condition: Event Name is blank
   
5. Set the Dues Years to Include as appropriate for your target audience (usually either current year only or all of the last few years).
   
6. Click Apply at the bottom.  Your grid data now contains the people who paid dues in the years you selected, minus the people who have already registered for the event.
7. Go to the Grid Data menu and choose Send Email