If you want to add one or multiple participants after someone already booked a time with you, you can do so as shown below:
1) On your Doodle dashboard, click on the menu with the three dots and then on "edit".
2) You will get to the calendar view where you can add and remove time slots if necessary. Click on the red button at the bottom of the page to proceed to the invitation page.
3) On this page you can add additional participants by clicking on "+ Add guests". Don't forget to send the invitation by clicking on "Send invitation". Only the newly added invitees will get the new invitation. You can also simply copy the link to the 1:1 invite and share it with others.