Updated for 7.0
On the left-hand side of the screen in the navigation bar under Member, select Messages. This will take you here:
In the menu bar at the top of the screen, select the button with an envelope icon titled Send Message To All. This will take you here:
Enter the subject of the message in the text box at the top of the screen under To All Member.
Use the text box directly underneath the title to enter your message.
After typing out your message, select the button at the bottom of the screen titled Send Message.
These are samples.