Sending a new post in an individual or group stream is quite simple. Have a look at the bottom of the screen where you will see the new post window:
Click into the new post window and it will expand to offer a few extras:
Here you can write your message (try mentioning people to notify them about your post!) and optionally give it a title, and/or attach images or other files. To learn how to format your post with text styling, click the "Formatting Help" link.
The button on the right will send your post to the group.