First, begin by putting students into groups. You can find articles that walk through this step by step here and here.

Next, click on the "Course Admin" tab and then "Announcements." Next, click on the blue "New Announcement" button

Next, fill out the required fields and scroll to the section called "Additional Release Conditions" when finished. Click on "Create and Attach"

Next, click on the "Select Condition Type" dropdown, then "Group Enrollment"

Next, click on the "Select Group" dropdown and select the group you want to send the announcement to and then click the blue "Create" button

Finally, click on the blue "Publish" button to send the announcement.