Just from a novices point of view, I might have each sales person refer people to a link with some GET-style data appended to the url. ei:
yoursite.com/url/to/page?salesPerson=Bob+Jones
when anyone lands on this url, you could grab the sales person in php using the good ol $_GET['salesPerson'], and from there, track or store it in a database or increment another counter of some sort. this would be basic, and you wouldn't necessarily know if a user proceeded through the whole sign up process on the site, but if you don't mind tinkering with the login block, you could make it so that data gets passed along to the landing page after registration where it could get collated there.

Thanks for your reply. The agents will be driving people to pages likehttp://www.site.com/tom andhttp://www.site.com/sally. Each of these pages will look identical to the user and then when the user had read through the site and thinks that they would like to signup they go to a common signup page. The signup page would need to know what page they originally came in from so that we could tell what agent was responsible for the visitor.

I hope this makes sense.

I happen to be working on an add-on that sort of does this! It will show who visited what pages, and for how long. This will show both registered and unregistered (guest) users!

Sounds great! When do you think it will be ready?