To send a private message another user, click on the chat bubble on the upper right corner of the screen and click "Open Messages" and then "New" to start a message.
Start typing the name of the person you want to message or click on one or people in the list that appears below. You can message one person or create a group chat with multiple people.
When you have selected who you want to message, type a message in the box at the bottom of the screen where it says “Write something…” to start the conversation.
You can also direct message an individual directly from their profile, by clicking the envelope icon below their banner image.