A database-wide chat is a single chat that holds every active member of your organization, kept in sync for you.
One chat for every active member of your database. Archived and offline profiles are excluded automatically.
Go to the Chats page, click the gear icon (Chat settings), open Database-wide chat, and turn on Enable database-wide chat.
Yes. Once the database-wide chat is on, a sub-toggle called Only admins can send messages lets you make it a read-only announcement channel.
Every active member is included. Archived and offline profiles are left out automatically, and the list stays current as profiles change.