As announced on the Facebook blog recently, Facebook users can now send messages to Facebook users and to email addresses outside of Facebook. The sending works the same way as traditional Facebook messaging, and the recipients can even reply to the emails you send. Most content you include in the messages will be included, but some may be excluded because of specific privacy rules (images, notes etc.).
This same process applies for events as well. For friends, family, or co-workers without Facebook, invitations may be sent to regular email addresses, but the discussion board is not available without registration.