@pfefferle I also had a look at federation between Lemmy and Wordpress. There is a minor fix I have to get in to make comparison of response content-type headers case insensitive (because you send charset=UTF-8 while eg Mastodon has charset=utf-8.
With that change I can already fetch Wordpress groups and users. But there is one more problem, Lemmy expects to find the Group that a post belongs to in cc or audience field. Is there any chance you could add that?