There should be a “share chat history” toggle switch when adding contacts to a group. When turned on, the app re-sends the past 10 messages (maybe the whole history), except self-destructing messages, to the new contact. The messages get cryptographically signed by the group inviter, not their original senders.
Currently if a contact is added to a group, they can’t see what happened in the group before they were added. Single messages can be resent by their authors to make them available to new group members; but they can’t resend messages sent by others (one of the reasons is that they can’t re-encrypt them to the new group member without destroying the signature of the original author).
This feature would help a lot with #10 - Onboarding: if a team member is added, add them to 5 recent relay groups and resend past messages for context - teams-bot - The 0x90 Git!, but would also be a general improvement.