You'll notice a News Forum (called Course Announcements) has already been added to each course site (top section). This can be used to send announcements to everyone enrolled in a Moodle course site. By default everyone in the course is "subscribed" to the News Forum so any announcement you post will also be automatically emailed to the entire roster. Also by default only Instructors or TA or Liaison roles can post ("Add a new topic") to the News Forum.
You can also add additional instances of the Forum activity for discussions (students can post to these!)