The Brotherhood of Senpai was originally a Halo 3 clan, however it is now a Halo: Reach clan. The clan was started by the users Ben Senpai, and Nate Senpai, however by the time that it was carried to Halo: Reach the clan was led by Nickel61.
The only five ranks are: Recruit/Soldier (BOS...), Squad Leader, (BOS...), Assistant Senpai (BOSZ), Senpai (BOSS), Head Senpai (BOSS)
The Brotherhood of Senpai is divided by squads. Squads are specialized groups of soldiers and recruits all under the command of a squad leader. Squads rarely have any members above the ranks of recruit, soldier and squad leader, and they never have Senpais.
A squad member has a specialized clan tag to show the squad they are in. Each squad is represented by a letter, therefore a squad members clan tag would be BOS plus the letter of the squad.