It's a handy way to get in touch with those individuals who haven't yet participated in your poll. To be able to send reminders you have to send out your invitations via Doodle, rather than sharing the participation link. Click the 'invite' button at the top of the page and then 'email.' Add your the addresses and hit send. This is also the way you can track who is missing. You'll see their email addresses listed as placeholders at the bottom of the table.
To remind those who haven't participated in your poll yet you can click the 'send reminders' button at the top of the page. Their email addresses will be listed in the box. You can keep the default friendly reminder, or you can personalize it if you like!