You can contact your classmates through the People link on the Navigation menu within a course, or through the Inbox link in the global toolbar (at the top right hand side of the screen), which takes you to the Conversation tool.
To contact a person within a course space, click on People (if your instructor has made this tool available to you). Click on the name of the person you wish to contact. You will be able to contact that person through any services they have linked to Canvas, or through the built-in Conversations tool.
You can also start a conversation with someone through the Inbox link on the global toolbar.
NOTE: Instructors have the option of inhibiting students from sending a Conversation message to all students in the course. If you do not get an option to select Students in the new message To: field, your instructor has turned that option off.
Also see the ELMS New Student Orientation.