Is it a bitcoin/crypto related group? If so, then you can make educational posts about exchanges, hardware wallets, tax/portoflio services. A lot of them have affiliate programs that you can apply to and earn from.
It's a health-themed type of group.
Whether the theme is Bitcoin or health, these are the common options,
1. Sponsored/advertising post.
2. Affiliate and referral link.
3. Accepting donation.
4. Renting group's banner/profile image, just like like member on this forum rent their signature.
5. Premium membership.
Although looking at this Facebook article
https://creators.facebook.com/making-money-on-facebook/, you or someone who asked your help can enable ProMode and check whether the group eligible for monetization.