If you are using Google Classroom to import your classes, you will need to update your Google Classroom roster before adding new students to your CommonLit class. You can add these students to your Google Classroom class on the People page of your Google Classroom class. For more information on how to invite students to your Google Classroom class, please see this Google Classroom Help article: Invite students to your class.
Once students have accepted the invitation to join your Google Classroom class in Google Classroom, you can then add them to CommonLit. To add new students from your Google Classroom class to your CommonLit rosters, go to your Manage Classes page. Then, click the Sync icon for the class you need to update. Then refresh the page after about a minute to see the new student added.
Any new students who are on that class roster in Google Classroom should appear on the CommonLit Manage Students page. They can then log in with Google and access assignments already assigned to the class.