How to message your clients through the dashboard
First, navigate to the "Users" tab from your root dashboard.
Next, hover over the thumbnail of the user whom you wish to message until the four small icons appear. Then, click the icon that looks like an envelope (second icon from the left).
The "Create Message" window will then appear.
Next, simply type your subject line and your message, then click "Send", and your message will be sent!
Note: It is not currently possible to "mass message" all of your users at once.