So far almost all the responses have focused on how an AS would end up in the BA. I think this is appropriate, after all it is a major factor in how the BA numbers could grow so large.

I'd like to raise another point, though, concerning how the numbers stay so large. The AS have, until tGS, denied or not acknowledged the existence of the BA.

Out in the world, if a villager suspects his neighbor is a darkfriend, he or she will say so. For jolly Joe Darkfriend there is a constant danger of being found out and there is a possibility that even the accusation will be enough to convict. If he shows even a single sign that something is amiss, the game is up.

BA AS, however, do not have that same level of danger. Since the AS publicly and to some degree privately deny that the BA even exist, I think they have a lower level of vigilance for suspected BA members. Due to either pride or willful ignorance, if an AS sees behavior in a friend that looks out of the ordinary, perhaps she'd suspect the friend is BA but then second-guess herself, and this doubt is enough to keep her from reporting the issue to an authority.

I don't mean to say BA members can't be found out. But I think the average AS might be willing to overlook a few suspicious behaviors.

A side point: the AS are also fiercely private, so it can be easier for BA to conceal whereabouts during secret meetings, murders, etc.

So given what everyone has said before about BA numbers being so high because of a possibly increased temptation to join the Shadow for an AS versus jolly Joe Darkfriend, I think we can also factor in a decreased danger of being removed from the BA (by being removed from life) versus jJD being removed from his ...cabal? (What would you call a group of darkfriends?)