Navigate to the main item "Groups", which you can find on your smartphone/tablet in the lower bar, or on your PC in the upper left corner. Now you can see all the groups in which you are a member. Select the group in which you want to create a post. On the bottom right you will find the "+ Post" symbol. A new window will open to create a new post. Now choose a headline and the content for your post. You also have the possibility to add files and images. To do so, click on the paperclip icon, which is displayed at the bottom.
Your post is ready and should be shared with your colleagues? Then simply click on "Create" in the upper right corner. If there is any small mistake, you can edit your post afterwards. Just click on the three dots in the top right corner of your post. There you can choose between "Delete post" and "Edit post".