If you are looking for ways to avoid using Facebook ads to promote your group, word of mouth is among the most effective ways to organically reach new users and grow your society.
According to some estimates, word of mouth is about 10 times more effective means of promotion than traditional paid advertising. It seems like users are indeed fed up with numerous pop-ups, intrusive banners, and ads that are swamping their screens (and lives), which is why word-of-mouth is expected to soon become the number 1 means of advertising and seriously jeopardize the already volatile reputation of rented spaces across the web.
How to generate word of mouth:
1. Make sure your business has something new to offer
2. Point out the problems you are able to solve and how you can solve them
3. Create a family-like community in which users will be treated with respect and where their voice will be heard
Word of mouth works best if those mouths are attached to the body of an influencer. Try contacting relevant and approachable influencers who have a good reputation and see if they are up for promoting your Facebook community to their (presumably many) followers. Of course, make sure you offer them something in return.