[Notify when visitor arrives and leaves area]

@Mak7ka Your script almost worked. I think this is a working version detecting correctly people leaving as well.

float timer_interval = 10.0; // how often to scan the area
list im_recipients = []; // a list of the keys of the people who will get IMed, if blank owner is set
list current_visitors; // keeps a list of current and previous people who were in the area, global values to preserve them between scans
list previous_visitors;
string region_name;

default
{
    on_rez ()
    {
        llResetScript();
    }

    changed (integer changed)
    {
        if (changed & CHANGED_OWNER)
        {
            llResetScript();
        }
    }

    state_entry ()
    {
        if (im_recipients == [])
        {
            im_recipients = llGetOwner();
        }
        region_name = llGetRegionName();
        llSetTimerEvent (timer_interval); // start the timer (and wait for it to trigger)
    }

    timer ()
    {
        list arrivals;
        list departures;
        integer count;

		current_visitors = llGetAgentList (AGENT_LIST_PARCEL, []); // get the list of who's here now

        count = llGetListLength (current_visitors);
        while (count) // for each current visitor (if any), working backwards through the list (beacuse it's easier)
        {
            key visitor = llList2Key (current_visitors, --count); // decrement the counter (to convert it to a list index, and also set it up for the while loop test next iteration) and get the visitor's key
            if (llListFindList (previous_visitors, [visitor]) == -1) // are they in the list of people who were here last time?
            {
                arrivals += llGetDisplayName (visitor); // if not, add their name to the list of new arrivals
                llInstantMessage (im_recipients, "Arrival of " + visitor + " at " + region_name);
            }
        }

        count = llGetListLength (previous_visitors);
        while (count) // for each visitor who was here last time
        {
            key visitor = llList2Key (previous_visitors, --count);
            if (llGetAgentSize (visitor) == ZERO_VECTOR)
            {
                departures += llGetDisplayName (visitor); // if not, add their name to the list of departures
                llInstantMessage (im_recipients, "Departure of " + visitor + " from " + region_name); // Note that avatar names not present can not be resolved
            }
        }

        previous_visitors = current_visitors; // save the list of current visitors for next time;
    }
}