Facebook Groups - How to Monetize?

Hi All. After some advice please.

I run a relatively successful Facebook Group. I started this group about 3years ago and it has grown organically from there. Its a 'Private' group (as opposed to Public) and has circa 13.5k members. The group is very specific to a high value product, which all members have a a vested interest in.

I administer and monitor the group on a daily basis - keeping spammers etc. at arms length. I have invested hundreds of hours in it thus far and the burden is growing as the group expands!

The group operates as follow; Members post a question on their topic of interest. The group 'Experts' then provide some open responses and post their direct contact details to follow up offline. The requirement for an open response is to educate other members who might be too reserved to post in the first instance.

Its at the stage now where the group 'experts' are doing significant business on the back of my group (think fishing in a barrel as opposed to a lake), and I am wondering how I can also benefit financially from the group?

I have considered setting up a limited company and invoicing these 'Experts' monthly but obviously don't want to fall foul of Facebook - which might result in loosing the group altogether! I would prefer to have Facebook onside, with maybe some relative banner adverts on the page and invoicing the "experts" on a monthly subscription basis.

How do I make this work for my family and I financially?

Thanks in advance for your comments/advice.