There actually aren't many orienteers who live inside Paris - most live in the suburbs and small towns around Paris. I can suggest that you try to get in touch with Richard Lazarowicz (map.image@wanadoo.fr) who is the contact for the club Raid Orientation Paris or Louis Bonfils (LBonfils@aol.com) or Serge Pons (pons2@wanadoo.fr) who are both in the Paris Orientsport club. These two clubs are the only Paris-based clubs and are quite small.
Another option would be to catch a train out to Chevreuse and get a lift from the station. Suggest that you try the club Guyancourt Orientation 78 (who are also organising the event) - their wedbsite is
http://www.go78.org/joomla/ and the contact is Serge Paulet (sergepaulet@gmail.com). There are English speakers in the club but I don't have their email address. Hope this helps!