Answer
Create new thread:
Click on
Groups in the top nav, where you will see a list of Groups. Click on the desired Group, and then click on the "Post Comment" button.
Reply to existing thread:
Click on
Groups in the top nav, where you will see a list of Groups. Click on the desired Group, and scroll down to the page to the Group's Forums. Click on the desired Thread, and click the "Reply to this Thread" button.