How do I add a co-teacher to my Google Classroom class in CommonLit?

There are two ways to join a class linked to Google Classroom:

Via the Google Classroom Sync button
Via the Join Class as Co-Teacher button using the class code

Option 1: Sync

In order to add a co-teacher to a Google Classroom-linked class:

Both teachers must be connected to Google Classroom (they will see a Reauthorize Google Classroom button on their Manage Classes page if they are).
Both teachers must be enrolled in the same official U.S. public or private school record.
The class must be enrolled in that same school.
The primary teacher must use the sync button (entering the class code will not work).

If your CommonLit class is imported from Google Classroom, here's the order of events that needs to happen to add a co-teacher to that class:

Teacher A (ideally the class owner) ensures Teacher B (the secondary teacher) is properly enrolled as a co-teacher on the People page of the Google Classroom Class.
Teacher A imports the class from Google Classroom to CommonLit on Manage Classes > Create a New Class > Import from Google Classroom.
Teacher B creates their CommonLit account, if it doesn't already exist, and connects it to their Google Classroom account by going to My Account > Connected to Google Classroom? > clicking the Connect button and connecting to their Google account. Once they grant CommonLit access to their Google account, they can exit out of the import process. They'll know they're successful if they see a Reauthorize Google Classroom button on the Manage Classes page upon refreshing it.
Teacher A and Teacher B make sure they are both enrolled in the same official U.S. public or private school (My Account > Schools), and that the class is also enrolled in that school (If the class needs to be switched into a new school, Teacher A goes to Manage Classes > Edit Class > School Dropdown).
Teacher A syncs the class from the Manage Classes page.

Assuming the class and teachers are in the same school and the secondary teacher has linked to their account to Google, the secondary teacher should be automatically added to the class.

Option 2: Join Class as Co-teacher with class code

If the above steps aren't working for you and your co-teacher, you can use the class code. Provide the class code(s) to your co-teacher(s) and instruct them to go to their Manage Classes page and click the Join Class as Co-Teacher button. They can enter one code at a time. This will send a request to your Manage Classes page for you to approve their request. Once you approve it, they can access the class as a co-teacher.