My usage of Yahoo groups in the early 2000s was mostly to communicate with my high school / college / dorm groups and the last thing I want is for embarrassing messages from 20 years ago sent to a private group to be archived.
And we're only archiving things that "any guy on the internet" can see. If someone can access the messages simply by joining a group (with no moderator approval), I'd argue it's fair game.
We're not going to be unreasonable, though. If something private slips through and we receive a takedown request from the author, we typically remove it.
* and by that I mean something that is not a telephone number, an address, a real life name, and other similar things.
If they have access to it, it is not private.