Type /whisper [username] followed by a message to send a private message to another user.
Example: /whisper [User 1] how are you?
Outputs: (Whisper) how are you?
You can send messages to multiple users at the same time by separating usernames with a comma.
Hovering over the "(Whisper)" text will display the users who can see the message.
Clicking on the "(Whisper)" text will allow you to send a new message to the same users.