/******************************************************************
* This example will hide a prim whenever an avatar is sitting anywhere on the object.
* Place this script into the prim you want to make invisible.
* Will hide the prim whenever an avatar is sitting. Just swap the llSetAlpha() lines to do the opposite!
******************************************************************/

default{
    changed(integer change){
        if(change & CHANGED_LINK){
            // if no avatars sitting
            if(llGetAgentSize(llGetLinkKey(llGetNumberOfPrims()))==ZERO_VECTOR){
                //make prim visible
                llSetAlpha(1,ALL_SIDES);
            }
            // if avatars are sitting
            else{
                //make prim invisible
                llSetAlpha(0,ALL_SIDES);
            }
        }
    }
}