just a warning for guilds going after the dark phoenix hatchling: the united nations achievement is likely bugged atm. there isn't a blue post to confirm/deny yet, however there are reports as recent as a couple days ago claiming that despite having multiple revered guildmates with UNIQUE exalted reputations, some guilds are still unable to complete this achievement.
it appears that the achievement is only taking into account a single revered guildie with the highest number of exalted reputations. this is NOT how the achievement was intended to work. (or else why bother make it a guild achievement, right?)
worst case scenario? this was totally intended and a SINGLE character in a guild must be the one to reach 55 exalted reputations while also hitting revered with their guild. this would probably result in an eruption of dissatisfaction and rage on the forums, though, so it's unlikely this potential bug will go unanswered for too long by blizzard.
i'm eagerly waiting on a blue poster to shed some insight on what may be the issue (if any), and if there will be a hotfix in the near future or will guilds need to wait until the next patch before seeing a fix.