I'm a member of several smaller groups but they are not monetized. In fact any promo is forbidden. For this reason the content is reliable and the community of people is strong. In larger groups you get an awful lot of spamming. As to whether or not people make money from It I couldn't say.
Most of the facebook groups are considered spammy and for spammers hence all the offers people auto posting in groups every few hours or so and to alot of internet marketers are considered a nuisance although yea there is some traffic there but your most probably competing with 20+ posts per hour really it depends on your intention in which you are thinking about using the group for the is also alot of professional groups out there no spamming aloud or members get kicked out
With the right, moderated Facebook group, you could have something good going for you. However, many Facebook groups are just spam and with all those people sending in ads every hour, there's little chance your own ad will be seen. Another thing is how most people just use auto posters for those groups, so it's not like anyone is viewing the ad anyway.
To me it makes sense to seek out the most important groups in your niche and join them as a productive member. If you're looking to spam links or such, a group is usually going to get pissed and you'll only alienate your target audience.
So, if you can't join a group and add a relevant, helpful post every now and again (with an appropriate link to your site or product etc.), I wouldn't even bother.
If you are passionate about the website/product you market, it should be ridiculously easy to write some posts every day to reach out to the target audience. But if you are just marketing for something you truly don't care much about and are simply in it for the money - you probably can't pull it off.