You can always add new participants to your group poll as long as you have not selected a final option yet.
To do so, find the Group Poll on your dashboard in your Group Poll tab.
To invite more participants using the invite link, click on 'copy link'.
To invite more participants via email, click on the 'share icon' and then click to invite via email.
Keep in mind, inviting participants via email directly from Doodle is limited to those with a paid Doodle subscription.