Upon looking to this further I'm not sure if it actually works as I understood it to, due to the way group services are handled currently in Mastodon. Clearly there is some sort of flag in Activitypub on group accounts to indicate to other apps that it is a group account, because e.g. https://lemmy.ml/c/climatejustice@chirp.social works and you can follow it but the same link substituting /c/ for /u/ does not work. And for normal user accounts, the inverse is true.

However, aside from that flag, my understanding is they are essentially just user accounts that boost any posts from followers that mention the account handle, which causes the boosted post to show in the feed for all followers of the account. Since that account isn't actually posting the posts that it boosts, I guess it makes sense that activity wouldn't be visible in Lemmy, where boosts don't exist. Following this logic no posts would be displayed, and that's what is observed. Initially I thought this was because no one on the instance had followed the group yet, because e.g. https://lemmy.world/c/BlackMastodon@chirp.social does show posts while https://lemmy.ml/c/BlackMastodon@chirp.social does not. The same group on a.gup.pe also shows more posts on https://lemmy.ml/c/BlackMastodon@a.gup.pe.

It's hard for me to make sense of what's going on here (especially as I don't microblog or use Mastodon personally) because clearly the Mastodon content is federating through the lemmy instance, but I've only been able to observe a subset of it and I haven't been able to figure out the parameters that have caused some posts to be visible in Lemmy but not others.