1. Affiliate Marketing (the easiest) - Join affiliate programs for products that genuinely help your group and solve some of their problems. Then, create valuable posts that naturally share those products as solutions, like product reviews. Ideally, find programs that are high-ticket or have recurring commissions.
This is a viable suggestion since a lot of e-commerce offers an affiliate commission by just sharing products. Your group will be best place to post health related products such as vitamins and other health care related equipment.
This method is very popular in my country especially if the group member is composed of organic users that discussing in daily basis. Many influencers and facebook group owners earned a lot through commissions even without directly handling the products from buyer.