If you want to send a Moodle message to the whole class or to multiple students in the class, here's how to do it:
- Go to your course's Participant's list.
- Select all participants by clicking the checkbox at the top of the left-most column, or pick and choose specific Students
- At the very bottom of the page, open the "With selected users..." drop-down menu, and choose "Send a message"
- Write your message
- Click on [Send message]
Your recipients will see your message in the messages icon (the little speech bubble icon) at the top of their Moodle pages.