Kinect quastion

Hi,
im using the open Ni plug in , I have to detect a single user and extract the rgb image from him (or her) , with OpenNi plug in detects different user but how can I isolate a single user ? whene there more people I cant identify single user !..
I made user + rgb data to blend fx from UNC on multiply to get the rgb data and black background …

please help …

anyone ?.. is that difficult to do ?