Byakhee are the classic servitor race, with human cultists a close second.
The Tcho-Tchos are counted among Hastur's worshippers.
There is also some kind of Cult of Hastur associated with the Yellow sign, that operates among the inhabitants of the underground caverns of K'n-yan. The Brothers of the Yellow Sign are organized against the Mi-go. The K'n-yan look like American Indians and are in possession of powerful psychic powers like telepathy and the ability to turn invisible and pass through matter. They utilize high tech and sorcery.
http://kinginyellow.wikia.com/wiki/Brothers_of_The_Yellow_SignThere are also the PALLID DANCERS that sometimes accompany the King in Yellow.
"Dressed in tattered rags to match their king, they
also wear small golden masks. Nothing is beneath
the mask and the wooden frame of a marionette complete
with cut strings is found beneath their rags."
The seem to be animated marionettes.
There are cultists who make "The Unspeakable Promise" to Hastur and become possessed by Hastur, transforming into monstrous figures-
"And certainly they saw the thing that came
crying out at us from the sinking ruins behind, the
distorted caricature of a human being, with its eyes sunk to
invisibility in thick masses of scaly flesh, the thing that
flailed its arms bonelessly at us like the appendages of an
octopus."
Finally there are some undead corpses that serve Hastur as it's eyes and ears on earth. Can't remember what they are called.