Only Contacts can be linked to a campaign. In a sense it is quite similar to the fact that you can only perform a Word mail-merge or send personalized emails to individuals, not Companies. There is simply no point in sending an invitation to 'Dear ABC Company'.
If you don't know the actual person at a Company you can create a contact called 'Sales Manager' or 'Managing Director' and use that Contact for the Campaign, e.g. when you send out the invitation for the campaign it'll be addressed to 'Dear Sales Manager'.
Later on, when you know the name of the person at that company you can simply update the 'Sales Manager' contact with the actual name of the person.