Most are logged into the system with their website which will give you a one-button RSVP link. However, most have actually RSVP’d on their websites and sent a webmention to that page for their avatars and details to show up on the page. (Here’s mine.) If you don’t have your webmention sender live yet, you can do it manually: https://telegraph.p3k.io/send-a-webmention.
If you want to go through the exercise and need some help, pop into the IndieWebCamp chat and we’ll help you get sorted.