Each class that you need to join has a unique class code. Ask your teacher what your class code is. Then go to www.commonlit.org/enroll. Enter your class code there. Then fill out your name, password, and a school email if you have one. You can also provide your personal email, or leave the email blank.
Have more than one class? You can add another teacher's class code by going to your My Account and Classes page. Click on Add another class code and enter your additional class codes one at a time.