While the actor is a Group and you can follow it, no posts are Announced. All the federation of posts is still driven by the individual communities within the feed. You'll need to modify Lemmy to add the logic of subscribing to the constituent communities when you receive an Accept.

Also there are Add and Remove activities sent out whenever the feed owner manages the list of communities within which would need to be handled.

Documentation still to come...

[–] nutomic@lemmy.ml 16 points 10 hours ago (1 children)

Ah its more complicated than I thought. We also have a similar or same feature on the roadmap, when I get to that it can federate with Piefed.

[–] jollyroberts@jolly-piefed.jomandoa.net 12 points 10 hours ago

For the federating its a new kind of AP actor. I'll be putting in a FEP for it in the near future, but its basically a "Group" that only cares about the "Following" collection.

You can see example json for the AP interactions here: https://codeberg.org/rimu/pyfedi/src/branch/main/docs/activitypub_examples/feeds

The AP interactions for a Feed are:

Send a Follow request for a Feed
Accept a Follow request (this is automatic for public feeds)
Reject a Follow request (this is automatic for private feeds)
Announce an Add of a Community to a Feed
Announce a Remove of a Community from a Feed
Send a Delete of a Feed to subscribers