There are different ways that you can make a profit from a Facebook group. The key is to understand your audience and know what type of services/products they would be interested in.

You can charge businesses or companies that are looking to target that specific audience a flat fee and post their products/services on their behalf.

You can add affiliate links for products or services that are relevant to your audience and earn commissions on the sales/leads generated from those links.

You can sell advertising on a CPC basis.

Etc...