In the messages area of a project, you can post a new message to the project by clicking the 'Add a Message' button in the top right.
You'll see a screen where you can add your message subject, content and choose:
- who to notify by email
- which category the message belongs to
- who the message should be private to
- any files you'd like to attach
- to add a tag
When you click 'Post Message' it will be added to the messages area of the project, and notifications will be sent to anyone you selected in the notification options.