This would be a new feature within the app. It would allow for messaging to and for subgroups within the Church community.
So, for example, there could be a section for small groups for each small group to chat, talk, discuss - what the next study will be, that the group wont be meeting, etc. The "Small Group Forum" could be accessible by anyone with the app, or password protected.
The section could be "open" and generated by any one in the church, or "closed" and only able to be made by Pastors and staff, etc.
Obviously anyone could comment on the forum, and just like the Prayer Wall admin would have right of removal.
If it helps, and you are familiar, think Reddit but for the app. A place for various servants within the community to orchestrate their efforts for the good of the community.