From what I know, you are required to join all thief guilds via "guild join yes". But since Tar Valon is now the Ruined City, it's impossible to join the guild, because the guilf for the Ruined City doesn't exist. On another note, if this is fixed and you can join the guild for the Ruined City, if it's ever rebuilt back into Tar Valon, will the donations/guild list/etc remain?