A facebook group is a community, it's like a forum. The thing that increases engagement is more content, the more the content the more people will engage. More engagement builds authority and trust, which will result in more loyal customers.
More engagement will make it a bit easier for a fence-sitter to buy, this is someone who's not sure if they need your product or not.
So before you even think about making money with that group, first find out what they like/want the most.
You can try a survey or simply offer them different types of content to find out what they like the most, then focus on that. You'll know what type of products will convert better once you know your audience.
Or else you'll be promoting product after product and see nothing or just a few sales coming in then end up giving up on something that could have easily become a goldmine.
So build trust with them and get to know your audience, that's what I would advise you to do with those group members.