In Settings, Global Settings → Groups, Users and Permissions → Sub-Organizations as shown below.
You will then be taken to a page that looks similar to this
This is where you will find your previously created sub-organizations, as well as where you can create new sub-organizations.
Click on the
+ NEW button
The following menu will be displayed
Enter the details of the sub-organization you want to create, the name and description, then click on the
SAVE button. You should then find your newly created sub-organization at the bottom of the list of existing sub-organizations.