1.Make sure you are using Mozilla Firefox web browse.
2.If you don't have then please download/instalit(http://userscripts.org/scripts/show/408961)
3.Login to facebook(www.facebook.com) if not logged in already.
4.Now open group where you want to add all your friends.
5.Now press CTRL+SHIFT+K it will open a Console Box.
6.Copy the given below code
7.paste into the Console Box. Then press enter, now wait for few seconds...(^_~) have fun!
