To add new students to your Google Classroom class on CommonLit, go to your Manage Classes page and press the green sync button for the class. Syncing will automatically create their CommonLit student account (if they do not yet have one) and add them to your class. They can then use the Log in with Google button to access their account.
Note: Syncing will only add NEW students to your CommonLit class. This will NOT remove students who are no longer in your Google Classroom.