This is an option for chat only implemented on the ST Community Server and ST Proxy Server. Basically the organizations are separated by using customized classes for searching and resolving users and groups. Results are reduced to the own Organization, so users of other organizations can not be found.
You can find some information in the ST 9 documentation
You do the configuration in the .ini and you can allow organizations "to see" each other or not.
Works fine for one of my customers.