Despite only being validated in February, I think Group federation has been a smashing success for the Fediverse.
I use Fediverse groups every day.
And if you’re interacting with this post, so are you.
Fediverse groups even make Mastodon that much more extensible despite Mastodon itself not officially supporting groups yet.
https://codeberg.org/fediverse/fep/src/branch/main/feps/fep-1b12.md
I was surprised to learn how easy it is to make a new group with https://a.gup.pe/ - just search for the group you want and it is is auto-created for you if not already made.