Two separate things: group chat, which belongs to a group, and direct messages between two members.
Group chat
Each group has a chat thread, and everyone in the group can post in it. The group's feed is different: by default only leaders can post there, and you can open the feed to everyone per group, which suits a small group and usually does not suit a large ministry.
The thread keeps its history, so somebody joining in March sees February. They arrive in a conversation rather than an empty room, which is a surprisingly large part of whether a new member stays.
Members can mute a group's thread, mention each other, and react to messages. A mention notifies the person even if they have muted the thread.
Direct messages
Members can message each other one to one.
Staff can't browse direct messages. There is no admin view of them; staff see one only when a member reports it. That is deliberate: a messaging feature the church office can read is one nobody uses for anything that matters.
That cuts both ways, and it is worth being clear-eyed about: you cannot supervise what you cannot see.
Reporting
In the mobile app, members can report a chat message, a direct message, a prayer request, or another member. Reports carry a reason (spam, abuse, sexual content, violence, self-harm, or other) plus whatever detail they add.
Reports arrive in Moderation. You can resolve or dismiss each one and record what you decided. Reports are listed newest first, and each shows how many reports there are against the same person, so a repeat pattern is visible rather than being four unconnected incidents.
Two things worth deciding before you turn this on
Who watches Moderation, and how often. An unread report queue is worse than no reporting, because a member did the right thing and nothing happened.
What you do about a self-harm report. It will arrive eventually, it is not a moderation decision, and the useful response is a person making contact within hours. Decide who that person is now rather than at the time.
Safeguarding
Direct messaging between adults and young people is a safeguarding question, not a software one, and this software cannot supervise it for you.
Whatever your church's policy is on adults contacting under-18s privately, it applies here exactly as it applies to phone numbers and social media. If you do not have one, this is a good reason to write it before enabling the app widely.