You can send a 1:1 meeting invite to multiple people so they will be able to pick a time and book a meeting with you directly. 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, but don't have to. Click on the red button at the bottom of the page to proceed to the page where you add participants.
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". If you do this, all new participants will get a 1:1 invite. The participants in they greyed out box will not get another email and can't be edited as they received an email the last time you sent out the 1:1 invite. You can also simply copy the link to the 1:1 invite and share it with others.