Earlier this month, Google released a great update to Classroom to help keep Parents/Guardians informed of class assignments and announcements for their students in Classroom.
Teachers can enter the Parents/Guardians email address in the Student section of classroom where it says to “Invite Guardians”. You should easily see if their email address is already in because they signed up in another class. Even if the email address is in, you still have to Invite them. Parents/Guardians will need a Google account too.
Here is a video showing how this works.