I would make a report with the police department as soon as possible. I doubt they will gain any kind of squatter rights in a short period of time, but they can definitely be a liability if they are injured or something happens.
Speaking of squatters, checkout this article on a Colorado couple that lost a million dollars worth of land to some squatters a couple of months ago:
http://www.latimes.com/news/nationworld/nation/la-na-land3dec03
- Tue Dec 4 2007, 13:51