Gangs or other groups of people have a huge affect on the world and what goes on in it. Sure, a motivated, experienced person can do some cool shit on their own, but it rarely compares to what can be done by a group, small or large, all putting their various skills and talents and efforts together to accomplish the same or similar goals.
In fact, that's generally the reason gangs and pretty much any other group exists in the first place. Yeah, sure, some groups may just hang out drinking, smoking, doing other such enjoyable activities. Even in such cases, the groups are still working towards a purpose. There are probably few out there who would argue with the fact that it's more fun to drink with other people than by yourself.
So, what are some more specific reasons that gangs exist, particularly in the Dome?
1. For protection. It's dangerous on the streets. And even a badass walking the streets with a shotgun isn't invincible. Sometimes even that guys gonna need back up. This applies even more to newer characters. The badass with the shotgun may need help ever once in a while but a newbies gonna need backup in a lot of situations, like when pisses off the badass with the shotgun.
2. To make money. Everybody's trying to make ends meet. So you find people who can help you out. Gangs sell drugs, guns, rob people, rob stores, run protection rackets.
Taking these into account, corps are basically well organized and funded gangs. They have security, they work with the law, thus there's protection. And they're centered around developing and marketing products. They pay their various members (also known as employees) for their contribution to producing and/or selling the product.
Gangs are often split into different sets and crews. This means that the gangs we think of as the Sinners, Snakes or Arteries are probably actually a group of smaller gangs who work together, wear similar colors, etc... That doesn't mean they're cohesive unit that always moves as one. They may work together one day to fight off another gang but then the next day they're set-tripping over drug territory. Similarly, corps may work with each other and they even have various departments. There may not be out and out fights between various corps or departments, but they don't always work completely in each others best interest.
The way I see it, there are two main gangs in the city of Withmore. There are the corpies and the mixers. The corpies are then divided up by what corp they work for and then what department within the corp. Mixers are divided up into several large gangs which are divided into smaller sets and crews.
By now, you may be sitting there thinking, "What the shit does this have to do with me?"
Well, I'll tell you. Find some other characters who might have similar goals as you. Want to be a drug dealer? Find a chemist and some muscle.
Want to be a ripper doc but got crap combat skills? Find someone to do the tough work.
Form a group with like minded people. Come up with a scheme and figure out what skills and talents are needed that you don't have.
Another thing to remember. In-game racism is completely acceptable. I don't support racism in any way, and I'm sure most of the admins and players don't either. However, that doesn't mean you character has to be the same. If I walk into Withmore not knowing a single person, as a white guy, I'm probably going to trust another white guy, before I trust somebody of a different ethnic origin. Don't be afraid to walk around the dome and when you meet someone with a similar ethnic origin, make that an excuse to talk to and connect with them. I'm sure every player out there could come up with dozens of reason for joining with certain characters to form a gang of sorts and I'm sure those gangs would have dozens of reasons both for working with each other sometimes and working against each other at other times. We're not one big, happy family. We're a bunch of smaller, dysfunctional, abusive families who can't stand each other most the times. Pimp your daughter, get your son hooked on crack, and the next time the neighbor kind tosses a ball into your yard, toss a fucking nade back.