Contacting users in your course can be done either on the Submissions Page or on the Course Management Page. Next to email subject and content, you can select which users to reach out to:
All users that are enrolled in the course
Only users that you manually select
All users that are enrolled in the course except for users you manually select to be excluded
It is also possible to email individual authors of submissions inside CodeGrade. This can be done using the Email Author Button on the page of an individual submission.