You must be logged into a JCPS account. Students: your school account. Parents: your student's school account.
Once you are in the link, you will be asked to join or create a class. You will join a class using the code given to you by your teacher. Codes are on teachers' websites who are using Google Classroom or teachers will be sharing those codes through email.
(If you have already joined earlier in the semester, you are already a member and can just go to the GC you were already using.)