Pie peeps occurs to me as a potential option
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Sorry, I thought the joke was implicit in your question, but the analog to "Lemmites" is "Piefites" which I think is just fine because say it out loud.
You know what, piefites is a good neutral option!
If there's a joke in saying it out loud or something my sleepy brain isn't finding it
"Pie fights"
Pieonaires comes to mind for me.
Oh how about Pioneers!
Crusties
(Pie)feds
Piefkes
Pie lovers but we are lemmings.
Glowies /j
Piessants.
Pieus
I'm also open to thoughts on "lemmites"! Its what I default to but I suppose it may be presumptuous for me to assume its what everyone uses, or wants to use :)
I have been using "Pie-heads", b/c we are just crazy enough to make that work! :-P

And the network of Lemmy + PieFed + Mbin (+ nodeBB + flarum) as the "Threadiverse", distinct from the wider Fediverse that also includes Mastodon, PixelFed, etc.
The fact that you name dropped NodeBB (a good example), Flarum (which has no working federation), but did not mention Discourse (which has only partially working federation and limited threadiverse support) gives me the warm and fuzzies 🤣
pieshiters
I got a better one if you understand french : pivert.
I don't really call them lemmites, haven't seen anybody else do so either, I think Lemmings has such a cool ring to it, very cute animals that travel in groups.
As for Piefed users if it's competing with something that cute then my vote is Piefolk.
Feds
Piefederates
Piefuckers
That's only for those on US-based instances... i.e. American Piefuckers
Winchesters

pie holes springs to mind
(to be clear i do not even remember what piefed is)