Customers managing their own Organizations

We are a B2B SaaS company and we use ZenDesk to provide support for the administrators at each of our customer sites.  Our Customers are in Organizations, and each customer can see the tickets for everyone in their organization.  We would like our customers to be able to manage their own users - adding and deleting people from their Organization as people come and go.  We would also like to limit the total number of Customers in each Organization to 4 or 5 people.  Has anyone ever configured their system for something like this?