It’s hard because you don’t want to be overly exclusive of beginners either. In the real world various mechanisms like cost, hiring, applications for membership, referrals can work. But online, unless you’re running a very small community, there’s way too much process and overhead in that, and it comes across as sanctimonious given it’s not standard practice at all.
Honestly I think one of the biggest reasons Reddit fails to work for experts is how voting affects visibility. When the majority of users are beginners, they’ll vote on things that appeal to and are understandable by beginners. Because voting affects the likelihood something gets seen, stuff oriented towards experts doesn’t get as much visibility, so discussion doesn’t happen, and it seems like that content isn’t there. In your example, at least there wasn’t a voting system making it so all the highest ranked questions at your meet up were from absolute beginners.
Somehow HN avoids this but I think it’s because they do both manual curation of bumping/upranking/downranking content and have some heuristic for vote:comment radio deranking posts. Plus low effort posts get banned or hidden.
Without the ranking:voting relationship, like in regular forums where ranking is just based on the last time a thread had a post, beginners would just ignore expert-oriented posts or be ignored if they contributed in a non constructive way. IMO that’s perfectly fine