Archipelago Mode in Bonfire!

Yes, it's exactly what you think it is (quoting)

The headline feature is Archipelago mode. The usual way to federate is "open": you connect with the entire fediverse by default and block bad actors as they show up, a never-ending game of whack-a-mole since anyone can spin up a new account or server. Archipelago mode flips this around: you can choose to federate only with a hand-picked allow-list of trusted people, groups and servers, and keep everything else out by default.

Trusted communities can in turn agree to federate with each other, forming an "archipelago" of like-minded "islands" with common rules and safety expectations. It's a great fit for tight-knit communities who only want to connect with their friends and allies, and for groups who federate together by choice (neighbourhoods, towns, assemblies, cooperatives, movements, or not-for-profit organisations) and want to stay connected to one another without necessarily being open to the wider internet. (Credit for the concept goes to Oliphant and Nora Tindall.)

https://bonfirenetworks.org/posts/bonfire-social-1-0-5-archipelago/