Organization/LegalAccount sioc:UserAccount (foaf:OnlineAccount)
I just had another look at sioc:UserAccount as explained in http://sioc-project.org/node/341
It has certain 'agency' limited to web/online resources, and have associated foaf:Agent (usually foaf:Person). In a similar way we could model Organizations - they would have 'agency' limited to legal activities and always have associated a real Agent (Person or Group).