Does group membership prove that a person saw every message?¶

No. Membership can show that content was available to an account under the service's rules, but busy or muted groups, background synchronisation and shared devices prevent an inference that the person saw every item.

Availability is not attention¶

A device may download messages without displaying them. Users can archive or ignore a conversation, and read receipts may be disabled or generated on another linked session. One person may not control every device connected to the account.

The volume and direction of the conversation matter: a direct mention in a short exchange presents a different evidential question from thousands of messages in a channel.

Look for message-specific awareness¶

Preserve mentions, direct replies, reactions, per-message states, notification records and linked-device activity. References to content, forwarding, screenshots, saved attachments or later conduct can corroborate knowledge.

State that the account belonged to the group or had access to a message unless the service event and surrounding evidence support personal viewing. Absence of a receipt does not prove unseen content either.