If you go to your group (which you can access by going to your profile) if you click on the 'Groups' Tab it should come up with a list of the groups that you are in. Click the one you desire to post a thread in! When you do, in the right hand corner of the screen just under the search bar there should be a big shiny blue button that says 'CREATE THREAD' in bold white letters. Click it, make a thread =D