You do not "join" the Space Marines. You are recruited out of populations of several trillions if you happen to possess the right characteristics, namely a near omnicidal lust for combat and the rough shape of a rather muscular flatbed truck. You also have to possess these characteristics as a child, becase adults tend to go completely insane from the transformation as opposed to only mostly insane. After being "recruited", you're put through several years of training in which you will, more often than not, die. If you don't, you're outfitted with several superfluous organs, genetically modified to make you an even more absurd specimen of manliness, and brainwashed into fanatically supporting the empire and its various conflicting ideologies. If you somehow manage to survive that, congratulations, you're a scout.