Hi @memokal , like @Gothix say it's not necessary to have an implement version about it.
But it's an idea interesting and i'll temper @Gothix answer (it's like family member system of ArcheAge)
It's more or less like an intern guild system, with little dev effort It could be something like this ( it's a 2-3 system tables and a interacting window ingame):
=> Table SecretHouse (Id, name, comment, create date, id tchat canal)
=> Table MemberSecretHouse (Id, id player, id SecretHouse, role on SecretHouse, title)
=> Table ExchangeSecretHouse (Id post, title, numberreply, Id SecretHouse, id player, message,...) optional
1 - A player could create some "SecretHouse" (ex: Galvanos followers)
2 - He could manage who is in by write it in the "SecretHouse" section.
3 - A tchat could be open on the "SecretHouse"
4 - A mini-exchange text post system ingame (optional) (because is data consuming)
This system could also permit some :
alliance-system ingame (managed by guild masters)
city-ingame tchat (managed by the city for city's citizen)
religion system (one by religion)
...