@grishka the way I do it in littr.me:

1. Aggregating recipients:
To: if the activity is in reply to another Object, I add that Object's author
CC: the explicitly at-mentioned actors
CC: the author's followers collection

2. Disseminating to local recipients.
From the list above I get all local actors. Things that are local collections are dereferenced and the local actors are kept.
The resulting list receives a copy of the activity in their inboxes.