The Pixelfed developer has been teasing Lemmy compatible groups for some years now. Apparently the code already exists, but for some reason it never gets merged.

At one point last year it looked like it would finally be released soon and I even set up a Pixelfed instance on my server to test it, but then somehow they decided working on Loops and Sup. was more important.

I got so annoyed by this lack of focus on basic functionality in general and a few other things regarding it that I deleted the instance again and do not plan to bother with anything this dev puts out again. Best of luck to them though, but I have better things to do with my time...