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Go to where your picture or your name is shown in the upper right corner
Chose My Groups from the drop-down menu
Chose which group you want to post in and click the group
The page opens with the Feed option already chosen for you.
Click inside the box and a pop-out window appears for you to write your post. You can also upload an image, video, or gif by clicking the icons
When you are ready to send your post click the Post icon