The Villain Society is an organization discussed in episode 85's Plain Old Mischief Makers which debuted 11 March 2013.
It may be related (an offshoot, a rename) to the Association of Villains that Granny May was going to get kicked out of due to an age limit of 75 in episode 47's Granny's Book Club which debuted 15 February 2010.
Some of the members (but not necessarily all of) include:
Tier 1 members
Tier 2 members
Tier 3 members
Tier 4 members
With a light-colored green background, these members are not "super-villains" like the first 3, they are merely "mischief makers":
These members were not seen on the pyramid illustration, but have been seen working for it (namely, intimidating BLHG and Bill). They are probably Tier 3 since they appear to boss around Tier 3s risking demotion to Tier 4, yet are still serving a menial role within the organization.
Due to some of the members not appearing on the pyramid illustration in Bill and Big Hand Guy's letter, it is likely that this was merely an illustration utilizing examples of people in the tiers, and is not the full roster of people in them, due to limited space in the drawing. As such, there may be other members in the illustrated 4 ranks. Guys like ARG are probably Tier 4,