You need to use the FTAF functionality that is available via a token in your email...It alows for the "forwarded" email to go out with decorated links specific to the net-new lead. Here is a link to the documentation:
http://community.marketo.com/MarketoArticle?id=kA050000000L8IBCA0
...The new lead will be tagged once they click one of the links in the email sent to them that results to a page with Munchkin...The good news is with this approach this new lead's engagment does NOT just get added to the activity log and score of the "Forwarder"...