It's not searching the contents of groups but rather the groups themselves. Using Facebook's search functionality it searches the title or description of the group for my terms. For example:

https://www.facebook.com/search/groups/?q=vape&ref=top_filte...

My iMacros bot visits that URL and it just goes through the results, parsing the data listed on that page.

Here is the code uploaded as a Gist: https://gist.github.com/accidentalrebel/79fda4536f799f4a4873...

It's been awhile since I last touched this so not sure if it is the lastest or if it still works.

Goodluck!