Mastodon

Group support with ActivityPub

Is freely licensed?	✅
Decentralization	federated
User interface	gui
Target audience	end users system administrators
Platforms	gnu/linux bsd
Website	https://joinmastodon.org
NLNet grant pages	https://nlnet.nl/project/Mastodon

Mastodon is the most widely used ActivityPub microblog — a federated alternative to X / Twitter. Communities can self-host (Mastodon docs; examples only—not a canonical list):

FOSS community
CCC-adjacent community
Social.coop community

Groups, richer filtering, and moderation tools sit on top of that base.