Increase facebbok Group Members

Change the button in the Ads section. You can try running a sales campaign, attaching the link to the group as a group link.

I find that running ads and attaching your group link attracts a lot of group participation and interaction if your posts or ads are good and attractive.

The "Join Group" objective is still being rolled out, so you might not have access yet. In the meantime, you can promote your group with Facebook Ads by mentioning it in the ad copy or using lead ads to gather sign-ups and invite people. Also, check out group management bots or Facebook’s Group Promotion tools. Engaging content can help boost organic reach too!

Try running a sales campaign, you can put the group link in the description and keep the group public so people can easily search