Attendees can send direct messages to speakers by clicking on their name and then clicking on the Send Message button. You need to ensure that all speakers have their public profiles along with the Send message feature enabled.
The first message someone sends to you is notified to you via email so you can go to My Messages and initiate a chat with that person.
If the chat window is already open, you're notified with a sound notification along with a text notification in the chat window.
