Technically speaking a Broker is anyone who provides the Earl with information or humans in exchange for money (and possibly other things), though some seem to make a living off of it, the one broker seen, a nun who worked at the Hearst Orphan Asylum, addressing an affiliate of the Noah Family over the phone as "Master".
Reasons for joining the Earl in such a way, presumably, vary, though the one Broker shown called those fighting against the Earl "selfish." She also seemed to be under the impression that, because she supported the Earl, she had gained some immunity against being killed by the Akuma, though she was quickly proven wrong when a Level 2 first turned her into a doll, then crushed her head and used her death to feed itself, showing that supporting the Earl as a Broker in no way guarantees safety.
Eliade's flashback shows that the Earl has a considerable amount of cellphones for each broker of his. Each cellphone is marked with a number , we can assume at sight that there is at least hundreds of brokers. 
- The Broker Nun of Hearst Orphan :Only one Broker has been seen thus far in the manga; one of the nuns who worked at the Hearst Orphan Asylum where Timothy Hearst lived. She was later killed by the Akuma, who had no qualms with finishing her off when she was in the way, even though she was a supporter of the Earl.
- The doctor at the akuma plant hospital: In Reverse and later in the anime a broker trades with the Millennium Earl and gives the souls of his patients in exchange of Money. He was talking them into giving in to despair and calling the Earl. He was kicked by Mother after being unmasked by Allen. His fate is unkown.
|The Noah||The Earl of Millennium • Road Kamelot • Tyki Mikk • Skinn Bolic • Jasdevi • Lulu Bell • Nea D. Campbell • Sheril Kamelot • Wisely • Toraido • Fiedla • Maushyma • Maitora • Allen Walker|
|Akuma||Mana Walker • Claire • Leo • Pierrot • Eliade • Eshi • Thread • Chomesuke • Lesley • Alma Karma • Mimi • Doug • Ice • Pang • Kazekiri • Sara • Doug • Sophia • Wizard Akuma • Rabbit Akuma • Level 3 (Paris) • Level 4 (Headquarters) • Level 4 (Paris)|
|Other||Dark Matter • Noah's Ark • Cell Roron • Lero • Teez • Akuma Egg • Skull • Brokers • Golem|