I wasn't able to watch most of the live letter but, forgive me if my information is incorrect, this feature will essentially allow up to 5 "roommates" to have housing controls just like if it was their own personal house.
Now I have several questions regarding this topic:
1) Has SE announced if shared houses will have private chambers like FC houses? I would think they would since 5 people can share a house together, but of course, it may not happen.
2) Have there been any talks on how payment would work? I would assume it would be the same as it is now but if you and a few friends or your spouse want to buy a property you'd just trade the percentage of gil you are putting down on the house to one person and they buy the house that way.
3) Has SE confirmed which patch shared housing would be released in and also have they said anything about housing wards in Ishgard?
Again, I haven't watched much of the live letter so all my info could be wrong on many levels.