We have discussed, how can you join a class, if the teacher has not invited you but shared class code in article Joining a Class. 

Let's find out how to join the class if the teacher has invited you in their class using Invite Students options.

If a teacher has invited any student, the student can find that invitation under Notifications. You can see your Pending/Accepted class list. 

Pending shows you have not joined this class yet and Accepted shows you the list of all the classes, you have joined so far.