As far as I can tell, it's entirely based on experience accrued. The more players a scout has, the more experience they can earn daily. I believe there's some sort of the rank that is affected by team ownership as well, but I have no proof of that off-hand.
From what I remember and some of my own intuitive thoughts, is it is a composite of player and owned team money earned. Also, if you GM or manage/coach a team you get a percentage of what that team earns.