By design, the Participant will not know that the Observers are sitting in on the session unless they are told (which we recommend for transparency). This is to make sure they are not a distraction to the Participant during the session.
As such, observers cannot chat with the Participants nor can Observers verbally speak to each other during the session. However, all observers and the moderator can use text chat to communicate with each other - as well as take notes - which will be time-stamped to the moment in the recording when it was made.