I have a custom list on a SharePoint 2010 extranet site which users from various external companies can add items to. The list's Permission Settings page allows you to make all items visible to all users, or make items visible only to their author however I need to limit the visibility of the list items to the person that created the item AND everyone else in their company too.

The company name is stored in Active Directory along with the user account.

Is it possible to achieve this programmatically, and if so, how?

FYI: New users will continue to be added to Active Directory long after the solution has been deployed (potentially from new companies) so I'd need a solution that accounts for that.

FYI: I don't want to create a seperate list for each company because there will be a large number of users and companies.

[UPDATE] Here's a simple example... Let's say Bill Gates and Steve Jobs have both previously added items to my list. Well I want all employees from Apple to be able to see & edit only Steve Job's items when they navigate to my list, and similarly, all employees from Microsoft to see & edit only Bill Gates' items.