First of all, DOWNLOAD SCRIPT
After downloading, locate and open the file then press Ctrl+C to copy the code, head over to the Facebook group which you want to add members to.
Hover your mouse to the left bar where you'll see "Discussion tab" click on it, then press Ctrl+F12
Disregard any warning you might see, paste the code right there and your members will be added just as fast as a jet. Enjoy