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You can message people in the room by clicking on their profile name and selecting "Message".
Messaging is available once you have selected a billing plan and provided your billing information, even if you are still on a free trial.
You can also message by clicking on the bell icon at the top right of the room, which opens a window with a tab for Direct Messages. There you can search and select a user to message, and also view your message history with that user.
Note there is a checkbox in the Direct Messages tab that needs to remain checked in order to allow others to initiate messages to you.