Completed all the Cataclysm Heroics.
Completed the Baradin Hold.
Clifford, being an Officer of a disbanded guild of the Burning Crusade expansion era, decided to create a new guild by the time of ToC, and so the Men at Arms guild was created, with a lot of players from the old Burning Crusade Guild.
The membership is open to players at all levels. Before the Cataclysm it was restricted, but now as every player may help the guild, Men at Arms is accepting all of them. Anyway, some restriction may still apply when creating a riding group.