I really like Val's suggestion.
In the past I have also just put in all my employees as contacts with the type "employee" against an account for my company and encouraged employees to always use their work email to sign up for things so I can dedupe them against their existing record. That way not only can I use the contact type field to filter them out of outbound campaigns, but I can also use Marketo's abilities in my favour and get an overview of what my employees are downloading, registering for etc and treating them like another target group.
Keeping your employee list in the system also allows you to run your internal comms from inside Marketo and track opens, clicks etc like any other communication. It can be very useful to track the effectiveness of your internal comms.