I was never in a gang, but pretty much everyone I grew up were in one so I know exactly how the initiation went. Long story short is you either get jumped in, trained in or 7UP'd.
If you're a male, you're getting jumped in. Basically the gang forms a circle around you and they just beat you up for a lil' while. More or less it was a ballpark of 3-5 minutes. You're allowed to fight back but rarely does anyone fight back, they usually just curl up on the ground and take the beating. There was a story of this one guy who was able to more than hold his own. Dude was like 17 years old or so but 6ft-something and jacked. The other teenaged/young adults were fighting him and not doing so well so they had to call the O.G. (Original Gangsta) from the sideline to humble the guy. You become O.G. after some years in the gang so he was a grown man. They called being jumped in, "proving your toughness". It made sense, if I was getting in a fight, I'd like to have someone by my side that I know can throw and take a punch.
"Running a train" is for the ladies. Basically to get a train ran on you means you bang each dude there. It lasts until everyone there gets a turn. It's some pretty degrading stuff but no one usually sees it that way. If you're in the neighborhood long enough to the point where you've chosen to join the gang, you probably banged a good portion of the dudes there by now anyway. It took place in a separate room so each dude there gets his privacy.
7UP is also for the ladies. 7UP basically means you're a gang member's girlfriend, you get to join because of him. No muss, no fuss. I read in this topic about how some dudes girlfriends got trained in, but they didn't do that in my neighborhood. I mean a guy's girlfriend is his girlfriend, you don't want to pass her around. To this day I have no idea why it was called 7UP and I did some Googling and can't find any other place that named it that. Maybe it was a stupid lil' thing only my neighborhood did but it'd be cool to know how they got the name.
Pretty standard gang stuff there.