We pinned informative topics in the respective categories; in my use case we had complex subject matter to discuss, so we broke it into separate topics to make it easier to digest and discuss.
Correct, the method I prescribed doesn’t work as well with private groups…
Taking it all together… maybe you could automate this workflow of yours, via Discourse Automation. Set it up so when a user joins a group, they get sent a PM with a link to the topic you want them to read…?