How do I post on a group forum that I have just joined ?

Here in the list of "message boards" you see when you go to community > message boards
you'll see a slot for "group forums" if you click the group name it'll take you to the groups forum page where you'll see a "new topic" button just like you did here in tech support.

If you instead go community > groups its at the bottom of the page when you pick the group from the display of groups you're a part of.

Thank you I was able to post :)