From the app home screen, click on the plus sign (+) on the bottom right of the screen.
The following screen will pop up:
You can share a post, link, image, or even choose your organization template.
By default, the post will be public and open to all the users in your organization. You can easily change the sharing settings by clicking on the top of the screen (see image below) and choosing the relevant option:
Public - Share and open to all users.
Private - Private note: Nobody will have access to it except for you.
Specific group - You can share it with a specific group with specific members.