Reach: 8800 members (January 2019)
The client provides copy and image for the post

I will schedule it in for you from my account (to increase the reach).
Posts that encourage members to comment will receive more exposure. For example, a giveaway with an entry mechanism via comments, is highly recommended.

A sponsored post in a Facebook group is very different to a standard advert. It needs to encourage comments or likes, otherwise Facebook won’t show it to many people.