If you're adding an external user from a new organization/workspace, they will see an empty universal channel when they're added.
If you're adding a user from an organization or workspace where the universal channel has already been created, they will see the message history when they're added.
Message retention settings follow the rules of each user's respective organizations.
For example, Sammy and Wes from Acme, Inc. are chatting in a universal channel. If they invite Miguel, a consultant from Globex, Corp., he will see the universal channel welcome message but no chat history. Only Sammy and Wes can see messages from before Miguel was added.
However, if they add Wendy, another consultant from Globex, Wendy will be able to see the message history, but only as far back as when Miguel was first added.
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