This means that the person's Messages setting does NOT enable chat with all attendees, and you cannot contact them using Messages.
Go to menu item 'Attendees', find the person you are looking for, click on their name, and Request vCard. If they accept your request, you can set up a meeting with them, and can now contact them using 'Messages' if their setting enables chat with connected attendees.
If you do not see the Request vCard button, this means that the person's public profile is disabled and they do not wish to be contacted via Dryfta.
If the person is a speaker, one final option would be to find their session, and send them a question or message in their session's Q&A space.
