Thank you @rimu@piefed.ngrok.app , now it's getting clearer in my head. I have added "https://lemmy.world/c/technology" , actual status is pending.

How long does this take? Is it an automatic process, or is it manually approved by an admin? So many questions...

It's automatic. But you might need to refresh the page before you see the result. Following an actor involves sending a follow activity and then waiting for an accept activity to be sent back. In Lemmy it should be sent almost immediately.

Some personal Mastodon accounts have manual approval of followers but that doesn't apply here. Still, the same communication happens under the hood.

nrw.social is a mastodon instance, which does not create group actors. Lemmy, Mbin and PieFed do. In Lemmy terms, a group actor is called a "Community". Every lemmy instance lists their communities at /communities, e.g. https://lemmy.world/communities.

Try putting "https://lemmy.world/c/technology" into that field and see what happens.

Yeah, as mentioned - Lemmy is often quite bad at sending the 'Accept' back for a Follow. It's particularly the case if the instance hasn't heard of you before, because in the process of fetching your user and instance details, it 'forgets' about the Accept. Within Lemmy, everyone's sort of got into the habit of 'Subscribe / Unsubscribe / Subscribe'. (where the first 'subscribe' lets them get your details, and the second one actually gets an 'Accept' back.)

PieFed is more reliable at sending 'Accept' back, so it's maybe useful to check your logic from this end with a community from there, e.g. https://piefed.social/c/antisocialmedia

I tried to sync @testing@chebucto.club with @testing-ground.

I did ask chatgpt, its new super familiar / friendly tone surprised me :ghost:

Looks like I was wrong before: whilst it's true that Lemmy isn't always great at sending an 'Accept' back, that's not the issue here. I was thinking about it from the perspective of a Person following a Group, not a Group following a Group.

It'll be interesting to see how the Lemmy devs respond. It's not really relevant for PieFed, because I'm not sure that there's communities there that have enough activity to warrant syncing to a category, but I thought about what would happen if you tried, and it would fail. The follower would be checked to see if they are a Person, and the request would go no further if not (there's no handy error returned, though).

I'm not sure it would be possible to enable, with the way the database is currently set up. We have a 'community_member' table, that looks like:

pyfedi=# \d community_member
                         Table "public.community_member"
      Column      |            Type             | Collation | Nullable | Default
------------------+-----------------------------+-----------+----------+---------
 user_id          | integer                     |           | not null |
 community_id     | integer                     |           | not null |
 is_moderator     | boolean                     |           |          |
 is_owner         | boolean                     |           |          |
 is_banned        | boolean                     |           |          |
 notify_new_posts | boolean                     |           |          |
 created_at       | timestamp without time zone |           |          |
Indexes:
    "community_member_pkey" PRIMARY KEY, btree (user_id, community_id)
    "ix_community_member_is_banned" btree (is_banned)
Foreign-key constraints:
    "community_member_community_id_fkey" FOREIGN KEY (community_id) REFERENCES community(id)
    "community_member_user_id_fkey" FOREIGN KEY (user_id) REFERENCES "user"(id)

So if the 'user_id' was actually a Group instead of a Person, then it would have a join to the wrong table.

I think some of the words, make ActivityPub more complicated to understand. Like what is an Actor ? This may be basic to some, but Im still not sure what an Actor is in this context, and why that word was used. Would be grateful if anyone can explain in one line :)