Acolyte(or Disciple, for members of the light side path), is the fourth rank in the Dark Brotherhood, which is awarded after completing the requirements of a Novice.
It is part of the Journeyman class, the fourth step down the path to Dark Jedi Knight.
Battleteam Leaders have the authority to promote up to this rank.
The requirements for the promotion from Acolyte are found at the above link, the completion of the above requirements will earn a promotion to Protector.