As an Administrator, you can message a user directly from your account.
Note: You can also use your Inbox to send messages.
Select the Managed Account in the Courses & Groups drop-down menu.
In Account Settings, click the Users link.
Find a User
Type the user's name into the Find a User field.
Open User Details
Click the user's name.
At the bottom of the page, find and click the Message button. By clicking on the button, you will be taken to your Inbox to send a message to that user.
Then, click the Compose a new message button to begin composing your message as normal.