As far as I know there's no way to download an official copy of Facebook groups, even if you are the owner. This is something that the Society of American Archivists wrote a letter to Facebook about in 2015 , and I don't think they ever received a response.
If you have access to the Facebook Group you may want to try using Webrecorder Desktop  to capture it as a web archive (WARC file). wenrecprder's autopilot functionality should let you automatically scroll the page to collect content.
I suggest the desktop version because you will have to log in to your Facebook account to retrieve the content, and if you use Webrecorder-desktop you will know that there is no chance of your credentials existing up on webrecorder.io
One nice thing about having the site as a WARC file is you can use it later to extract images and things like that from the WARC. It might be a bit thorny, but at least it's possible.
> On Nov 4, 2019, at 11:25 AM, Carol Kassel <[log in to unmask]> wrote:
> Hi all,
> I wondered if anyone here has any experience in preserving the content of a
> closed Facebook group. I thought it would be straightforward to create some
> kind of archive, or download all the photos, or really anything. But it
> isn't. Searching for a solution yields a variety of interesting options but
> I'm not sure what to try.
> Please note that I used to write commands but now I'm a manager-type who
> only issues commands. However, I can roll up my sleeves and try to remember
> what I used to be able to do, if that's what it takes.
> Thank you!
> Carol Kassel
> Senior Manager, Digital Library Infrastructure
> NYU Digital Library Technology Services
> [log in to unmask]
> (212) 992-9246