How to see who the individual is who registers multiple people for a paid event
Results from CiviCRM demo (4.5 on Drupal)


Additional participants do not have a contribution source, so they can be identified on a Event Participants List Report (you will need to select the Contribution source in the report criteria)
Click view on the participant and "Registered By" will be shown.
Clicking the "Registered By" participant will take you the that person's registration showing all the people he/she registered.