Navigate to the Menu and click on Organisation and Users.
Alternatively, you can access it via the User Admin link in the Common Tasks portlet on the homepage.
In this example, our organisation is called Bank of Unicorn Limited and it is organised like this:
So, in the above example, 'Investments' and 'Mortgage Services' are parent units and 'Corporate Investments', 'High Risk' and 'Private Investments' are child units of Investments.
On the right hand side, you can see how many users there are per unit. So, Investments has 9 and Mortgage Services has 5.