Fortunatly I have a boat big enough to put extra people on (Stiletto 27) Speed is relative, when you get a boat that big moving good it impresses the heck out of people, sailors and non sailors alike. You then hand a sheet over and show them how to trim for starters. Its almost a given that they ask "whats next?". My favorite is giving them the helm to let them feel what is happening when the sails are set and the boat takes off. The down side to this method is "hey you've got room, why do I need a boat?" The upside is possibly they will want to sail more and possibly faster and get smaller, more portable beach cat. Either way someone new is sailing.
I also work with my sons boy scout troop (as the small boat merit badge counseler). Now thats the age to hit them. They really have a need for speed!
Clayton