Automatically add people to facebook group

I have been added several times to facebook groups I do not even know. For curiosity I went on them and saw that some of these had even over a million group members. The only explanation for me is that they use some sort of robot to add members to these groups. How do they do it? Can you develop a mechanism that can add people to my facebook page without someone manually being involved? I can provide a screenshot of that group that added me and has hundreds of thousands of group members. I have a list of contacts but adding/inviting them manually would be too tedious. Thank you