In order to create a new discussion post, please make sure that you have joined the group. If you have not joined the group yet, please click on the "Join" button on the right-side menu on that group's homepage.
How to start a new discussion post:
When on the group homepage, click on the + New button at the top right of the screen and select "Start A Discussion":
Fill in all required fields on the next screen and then select the Post button at the bottom.
Alternately, you can select Discussions from the top navigation menu on any group's homepage. When on the Discussions homepage, you can select the + New button at the top right of the screen and that will open up the new discussion page.