The Brotherhood of Grey Jedi was founded in 16 ABY by four dissident Jedi from the New Jedi Order. The original members were Jedi Knights Zela Kssir, Yesew Thrunner, and Garru Joster, and Jedi Master Alli Gevattu. They took a Skipray Blastboat to the planet Baroonda, and founded their Brotherhood there.
Within months, they had increased their numbers to 14, with the discovery of ten Force-sensitives in the local populace. They taught them the ways of the Grey Jedi: that the Force had two sides, and only the strongest could walk the knife edge between them, resisting the pull of both sides, and still harness both sides' abilities. The New Jedi Order believed the four original members to be dead, so they did not discover the Brotherhood's existence until around 18 ABY, after the Grey Brotherhood helped found the Baroonda Consortium.
The Brotherhood had 4 ranks of their members. The beginning rank was called a Pupil, and was similar to the Padawan level in the Jedi Order. Next came two ranks that were equal: Protector and Scholar. Protectors were Grey Jedi who wore battle armor and went to war with the Marines and Navy of the Baroonda Consortium. The scholars were Grey Jedi who sought to uncover the many secrets of the Force. When a Protector or Scholar reached the level where he or she had mastered the ability to walk on the knife edge of the Force, then they were elevated to the Craaft level. This was similar to a Jedi Master in the Order, as they helped to make important decisions, and voted on two Craaft rank members to sit on the Council of Six for life.
Each rank had a certain color of lightsaber that they carried. Pupils had grey-white sabers, Scholars had yellow sabers, Protectors had orange sabers, and Craaft usually had navy-blue lightsabers, although some chose to keep their old sabers from their Protector and Scholar days.
The Brotherhood had a very different stance on the Force compared to the Jedi Order, the Sith, or even other Gray Jedi organizations such as the Imperial Knights. Their view was that it was easiest to give in to the pull of either the dark or light side of the force, and to do so was dishonorable. They believed that the true path to becoming a master of the force was to succumb to neither darkness or light, but rather to walk the thin line between them. This was the aim of the Brotherhood's Grey Jedi. They harnessed powers such as Force Lightning or Electric Judgement, and in battle they would use their anger against their foes. However, such things were often counterbalanced by Jedi-esque meditation after a fight.
In the Baroonda System, the Grey Jedi were treated as heroes. They took an active part in government, composing 1/3 of the Council of Six, and even fought wars alongside Consortium troops. However, they did not take much of a part in galactic policy-making.