People within the same company can't necessarily see all the tickets from that company. The default should probably be that they can.
Within Admin, Customize your Public Customer Portal
Have options for, e.g.;
'Users can view all tickets raised by their company'
(i.e. users can see all the tickets raised by all other users listed under their Company)
'Users can view and contribute to all tickets raised by their company'
(i.e. users can join the thread of communication for all the tickets raised by all other users listed under their Company)


