By default, users who are not in a group cannot retrieve its history messages. When this feature is enabled, users can retrieve history messages from a group even if they are not a member.
To enable this feature, go to the Basic Features page in the developer console, confirm that the correct app name and environment (Development/Production) are selected, and then click Enable.
Related documentation: Retrieve Historical Messages | RC Developer Documentation
If you have any questions about this feature, feel free to submit a ticket to us.