Re: Joining the Brotherhood, the only faction that cannot join is Cybertronic (though it is rarer for Mishimen to join due to the schism). The joining in later life rules are covered in the quote below. For more on both, see page 96 of the Brotherhood book:
"
JOINING THE BROTHERHOOD IN LATER LIFE Many people join the Brotherhood during their adult lives, and the
entire population of Lay-Brothers is recruited in this way. Lacking
the long and arduous training of an apprenticeship, and the raw
aptitude to wield the Art, these initiates fill vital roles that do not
require mystical prowess.
At the start of any Career Phase, before rolling or choosing a
career, a character may choose to join the Brotherhood. The
character replaces his Citizen (or Under the Radar) talent with the
Book of Law talent, and the Brother talent (page 249 of the core
book). From that point on, the character is considered to be a
member of the Brotherhood, instead of a member of his original
faction. Characters with a Cybertronic heritage cannot join the
Brotherhood."
Just another Ghost in the Machine