A 1.9 thread is a player creation. Most such threads are a members only Guild thread.
There are some multi party threads, aside from Guilds, but to take part you mut be either a Guildmate of Friend of the other players.
Neighborhood also counts but only when in the same hood. so no one allows hood relationship to matter in a 1.9 (has to be Guild or Friend)
The reason you cannot place anything in the thread you have gained access to is you were not a friend or guildmate of the person who owns he Great Building you wanted to add FPs to.
The thing to do is click on the Great Building owner's avatar, and click on 'add as a friend' .. If they accept ... then later you can add to thier GB.
(it may only take minutes, or a day or more for them t o accept your friend request. Only after they accept can you add to their Great Building.