Hi,
I would suggest you can use team concept or access team or queues or using workflow you can assign automatically to particular business unit team.
http://crmbusiness.wordpress.com/2014/05/15/crm-2013-mb2-703-access-teams-and-access-team-templates-how-to-use-them-and-key-facts/
http://blogs.msdn.com/b/crm/archive/2013/06/13/using-teams-to-solve-complex-record-sharing-scenarios.aspx
http://www.microsoft.com/en-us/dynamics/crm-customer-center/find-what-s-assigned-to-you-in-a-queue.aspx
https://community.dynamics.com/crm/b/rockmycrm/archive/2013/06/24/working-with-queues.aspx
Thanks & Regards, MS CRM Consultant, V.Surya. My Blog: http://inventcrm.wordpress.com/