Stay Classy

Easy way to get this, but it takes a few steps

According to WoWPedia.com

Patch 5.0.4 (2012-08-28): "Guild reputation doesn't automatically reset to Neutral on joining a new guild, now only drops to next level down, Exalted->Revered, Revered->Honored, Honored->Friendly, Friendly->Neutral."

Step 1. Find that Race-Class combo you are missing. (ex. /who Pandaren Hunter)
Step 2. Make sure they are the correct level. (pandaren 90 all other 85).
Step 3. check the Armory, make sure they are guild rep exalted
            (Achievement credit is at Honored so it should work if they are Revered)
Step 4. whisper them and I offered some gold usually 3k-5k for their trouble.
Step 5. invite to your guild then have them log out and in and DONE!
Step 6. pay 'em and Repeat

You stay classy, Orgrimmar.

For those who are wondering, it requires a minimum guild size of 46. The sum is:

7 orcs (hunter, shaman, warrior, mage, rogue, warlock, death knight)
9 trolls (hunter, priest, shaman, death knight, warlock, mage, rogue, warrior druid)
8 blood elves (hunter, priest, warlock, death knight, mage, rogue, paladin, warrior)
7 tauren (druid, shaman, death knight, priest, hunter, warrior, paladin)
7 undead (mage, rogue, death knight, hunter, priest, warlock, warrior)
8 goblin (mage, rogue, death knight, shaman, priest, warlock, warrior, hunter)

Comparatively, Alliance only needs 45.