Hello,
The fastest solution is to use Account Manager field (that is added by CRM optional component) for this purpose.
To achieve that, you need to go to Menu->Administrator->Record Browser, select Contacts and switch to Permissions tab.
Now, edit the "View" rule for "Employees". Change the rule so the Criteria required are as follows:
Company Name equal records with Account Manager set to User Contact or Company Name equal User Company
The first bit allows the user to view all Contacts assigned to the Main Company, as you requested. The second part is to enable your Employees to see other Employee's contacts in the system.
Hope this helps,
Arek