Staff members are chosen by the staff.
We have a section dedicated for looking out for future staff members, and we always keep an eye on members doing well/helping out/showing knowledge of the forum.
When the staff needs a new member, we pick who we think can do the job the best. It's not "the best member" but "which member can handle x y and z based on posts/actions on the board".
Also, we do have super cool hidden forums to have fun in.