The 1-on-1 chat feature can be used by event participants, moderators and speakers, and you can send messages to all event participants.
The chat and its history work independently of the activity / session in the Engagement app. This means that you can keep a conversation going regardless of the activity / session you are currently in, in that event.
You can start using the chat by clicking on the 1-on-1 Chat tab and clicking on the + button to begin a conversation.