If your router supports SNMP, you can use this app to monitor total traffic. Before you buy the app, use the same vendor's free SNMP test utility here to make sure your router supports SNMP.
Apple's routers do not support SNMP if that is what you happen to have.
I am not network engineer, but I do not think that a simple app will accomplish this unless all devices connect (and give you a breakdown of each device) to the internet through your Mac (via internet sharing). If you are trying to monitor the whole wifi network for multiple devices you need to monitor it at the wifi router level. Check out the article here: http://lifehacker.com/5917367/how-can-i-find-out-how-much-bandwidth-im-using-at-home