One of the most important aspects of running a prosperous FB group is treating your members like your peers. Interact with them on a daily basis, ask for their feedback and come up with an effective incentive program through which you will be rewarding your most active and most valuable users.
This way they will feel like they are truly getting something in return, and will feel like relevant members of a great community that indeed works towards something better.
This type of motivation will work wonders for the growth of your group, and these top members will soon become priceless advocates of your business/agency/product/brand, so make sure you (or your admins) are not reluctant or (even worse) too lazy to interact with your audiences via likes and comments. Ideally, you should be able to provide answers to ALL of the questions your members ask and try to be as swift as possible.
Also, make sure you know WHEN to post and engage with them. Timing can be vital here, so conduct a little research and find out when exactly during the day your members are most active and try to post your best and most engaging content then. General rule of thumb is that most Facebook users browse their news feeds around 11am and 7pm, however, this information depends highly on your target audience.
If you cannot manage your Facebook community consistently all by yourself, try hiring people (admins) who will be able to keep it well-organized and alive by consistently posting, commenting and sharing. Note: You can also use a social media scheduling/managing tool for these purposes, like Social Lift).
Reference: 7 Best Practices To Skyrocket Your Facebook Group, By Branislav Bozanic, https://planable.io/blog/facebook-group-best-practices/