You could use an ethernet shield and instruct the arduino to email a group of people.
Or a GSM shield and it could SMS those people.
Zigbee/xbee are more for device to device communication, for example you could have an arduino and rubbish sensor at the bin communicating over xbee to another arduino which is connected to the ethernet/a computer
hey thanks for your really long and helpful input down there at the topic just want to ask a question.. what's the difference between the xbee shield and the xbee module such as XBee 1mW Wire Antenna? the arduino and xbee will be tgt and send out data to the computer... so which be more appropriate to use?once again, your reply is much appreciated!!