The listing of Contributors is a Page rather than a post. Just log in, click the Contributors menu and, once the page has loaded, click Edit This on the page itself, just under the title. The page editor will load.
The list of Speakers is alphabetical so just navigate to where you want to add a new entry, hit return and a new bullet point will appear. Enter the speaker’s name and make it bold using the formatting menu. Add a biography (use the existing examples as a guide). Then click Publish in the box in the right column.