New method of steps for messaging participants as a teacher via the participants area in a course: 

To send a message to selected course participants (for teachers only)

• Click on the Participants link in the People block on the course page.
• Select participants from the list or use the "Select all" button at the bottom of the list.
• Choose "Add/send message" from the "With selected users..." drop-down menu.
• Type the message in the pop-up window
• Assuming you are satisfied with the message, click the "Send" button. 

https://www.screencast.com/t/AHIiO8R2

Old method of steps for messaging participants as a teacher via the participants area in a course: 

To send a message to selected course participants (for teachers only)

• Click on the Participants link in the People block on the course page.
• Select participants from the list or use the "Select all" button at the bottom of the list.
• Choose "Add/send message" from the "With selected users..." drop-down menu.
• Type the message then click the "Preview" button.
• Assuming you are satisfied with the message, click the "Send" button. 

https://docs.moodle.org/27/en/Using_Messaging#To_send_a_message_to_selected_course_participants_.28for_teachers_only.29