In Friendica we already do support public groups. We do it this way:
- Person A is posting a public post that is directed via “to” and mentions to the group B.
- The group is checking if person A is a follower of group B.
- If yes, then the group is transmitting an “Announce” activity to the followers of the group B.
Additionally we do know different address forms. We can address the group via “@”, then the post is transmitted to all followers of A and to the group. But we also do know the addressing via “!”. If that is done, the post is still public, but is transmitted only to the group.
This working fine on public groups, indeed you should be able to interact with the group with nearly every AP implementation.
But: Friendica does know private groups as well. These posts are non public. This means that it isn’t possible to send some “Announce” message, since the original post can’t be public. But when it isn’t public, there is no way for the followers to fetch them.
I found the “proxyUrl” value. Possibly this can be used? But how? Or do someone knows another way?