Note: You need to create a group in your course and make sure that you belong to that group.

A teacher can send a message to course participants as follows:

Step 1: Go to the Participants page.

Step 2: From the filter drop-down menu select the group you wish to send a message.

Step 3: Tick the Firstname/Surname check box to select all the students.
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Step 4: At the bottom of the page choose "Add/send message" from the "With selected users..." drop-down menu.

Step 5: Type your message. Assuming you are satisfied with the message, click the "Send" button.