According to numerous sources (WoWHead comments, Warcraftpets.com, etc.), this achievement is currently completely broken. Instead of requiring a total of 55 unique exalted faction rep from all your guild members, it only counts the highest faction rep of ONE member. In essence, the way the achievement was coded into the game, it was designed to be a SOLO achievement instead of a guild achievement.
If your guild is working on building unique faction reps separately, they won't count until they fix this bug. Teni (talk) 22:50, 10 March 2011 (UTC)
While 4.1 was on the test realm, the patch test notes included this:
Patch 4.1.0 (2011-04-26): The guild achievements for earning exalted reputation with factions (Ambassadors, Diplomacy, and United Nations) have been fixed to now correctly count cumulative guild members with exalted factions. Previously, these achievements would only count exalted factions on one guild member.
However, when 4.1 went live, this note was removed from the patch notes, and as of today the achievement is still broken. Shoutingboy (talk) 20:19, 27 April 2011 (UTC)