To Send Message to users, click on Manage Users --> Users, the page as shown in the screen here is displayed:
Select the users and click on Send Message button as indicated by the red arrow in the screen above. This action will open a pop-up window as shown in the screen below:
Note: This message will be sent to those users who have 'view messages' privilege and preference
In this pop-up enter the Subject and Message to be sent
To send the message, click on Send button. This action will prompt a confirmation message in a dialog-box as shown here: