Event Participant invitation
Search for Contacts w/ specific criteria (if you like to narrow things down) -> then add the resulting Contacts to a Group; make sure in Manage Groups that the Groups is a mailing group; Then go to CiviMail and email this Group your Event details + link to registration page.

If you have fewer than 50 Contacts you can also send Email from the Result of the Contact search screen; over 50 you'll need to use CiviMail;

If you want to get really fancy -> embed a checksum token in your Email message to your Contacts -> that way when they get to the Event registration page existing information (already in your CiviCRM) - will be filled out for them;

The details on how to use this would depend on your CMS -> but here's an example for Drupal (where IDNUMBER is the ID of your Event)

http://www.myorganization.org/civicrm/event/register?reset=1&amp;id=IDNUMBER&amp;{contact.checksum}&amp;cid={contact.contact_id}

When using checksum tokens -> they only have limited lifetime (for security reasons) -> you may need to visit Administer -> System Settings -> Misc -> and change the Checkum Lifespan (7 days is default);