BentiGorlich , 4 hours ago

AP has a tool for that called inbox forwarding and mastodon uses it for sharing the comments under posts. It works like this: you send a reply to a user with their follower collection as the recipient. You of course cannot know who is following that user, however they than just forward this reply to the follower collection, because the server knows that it has authority over that collection. https://www.w3.org/TR/activitypub/#inbox-forwarding