Depends on the legion/chapter.
Like in the "Blood Gorgons" novel, they have a gene-seed and use it to create new chaos marines (which they harvest hardy humans off of a planet). They bond these new brothers through organ implants and "Blood Bonding" which links an older member of the chapter, and a new initiate.
Really cool, even though the exact process isn't described, but the Gorgons seem more like the Space Wolves and Blood Angels Codexes, they still act and are organized much like loyalist marines, but the taint of chaos is there...
Definitely recommend picking it up