You can message others in the interactive class using the chat feature which is located in the tools at the bottom of the screen
You can use the chat feature in the following ways:
Clicking on the chat icon in the tools will open the chat panel. Here you will see all the messages posted by other participants and you will also be able to add your own message.
Click the fullscreen icon in the bottom right of the video player to expand this video -
Hovering over the chat icon in the tools will allow you to enter a message directly by typing as shown in the video below