The service overseer sets up the list and he may ask for input from the other elders. I have seen the SO set up a list without any input or little and he had people who gave rides to people in separate book studies from the people they gave rides, he scheduled older JWs that couldn't handle stairs to book studies in the basement, he scheduled sisters in the same book study that hadn't talked to each other for 20 years, he scheduled a family known for thieving children to a private home, he scheduled a book study with people 70% of whom hardly put in 2 hours a month and rarely attended the book study, get the drift.
Then once the list was posted in one congregation and except for the conductors and their assistants, and the family who lived in the home, no one went to the one they were assigned to.
I knew one family made up of grandparents, grown son and family, grown daughter and family that would not be split up. That made 12 people that had to be assigned as a block.
Currently it seems very hard to find JWs who want the book study in their home if the meeting for frield service is held there. So the book study during the week is in the home but the group meets at the KH for field service.
The sheep are not very obediently or willing for the "privilege" of having the book study in their home. Also many JWs do not have a house big enough for a group...probably because their not making big bucks due to the limited education.
Blondie