Adverse position (acquiring property ownership by squatting) IS legal in all 50 states, so check your state's requirements to see exactly what is required from a time standpoint to claim ownership. Wikipedia has useful information. I believe from what I have read that California has the fewest number of years, about 5, with 20 being about the most. Note that you can circumvent the claim by giving permission to the squatter to use your land. I personally feel that if you are okay with allowing them to use the land that you should send them a certified letter with a signature required to document that you have given permission. Otherwise you could lose your land to a simple court filing. Your alternative is to forceably forbid their use of your land, or simply draw up the documents to transfer title.