Joining a conversation course is very helpful for the oral exam but the Chat Club is not directly a prep class for the exam. However you will practice vocabulary and topics which you need during the respective Goethe exams. You will also learn how to master presentations and practice dialogues during the class. You will also get set phrases, which you can use in the exams. In addition to that you get live corrections from an experienced native tutor.
So it will be very useful as an addition to your exam prep.