Set up an Element room with test accounts on different devices and, if relevant, different Matrix homeservers. Send messages, attachments, and an invitation; then check history and access after a user leaves. If encryption is enabled, rehearse verifying a new device and recovering keys.
Decide who administers rooms and how moderation, retention, and backups will work. Messaging is a dependency when incidents happen, so test login and access recovery before moving critical coordination to it.
Akousa