I think you either have the wrong idea about RFID tags or you don't really mean "locate" people?
Have a look here : **broken link removed** and here : **broken link removed**, but this is far more complex than the basic passive RFID tag, which will not allow enough range to do what you're looking for...
As far as I know, RF ID tags is more like a unique radio station identifier. It is a way to determine which transceiver is which.
You will need to get a hold of the person first, and hand them a transceiver. Assign a channel to him/her, then get another person, and assign a different channel to him/her.
Then if that person takes off forever, you will know how to contact the person, but you won't know exactly where he/she is without them telling you their location.