The functionality to send group chat messages during Events is available to Hosts and SALT users only.
Anonymous users can view the chat, however on trying to send a message they are prompted to register for a SALT account, which only takes a few seconds, before sending their message.
To send a message click on the speech bubble icon at the bottom of the screen and from there you can type your message and hit send. In this window you will also see all messages sent during the Event.