A good way to make people stick together is to make it not so competitive when spawning in a local area. Plenty of resources around camp, makes it so people will not kill each other pointlessly. And those that do, will be ostracized by the local community. Also the clothing you craft using local materials will distinguish you from players from other "camps" farther away in the map. So when you wander the map and see others in the different clothing (because they spawned in areas different from you using different materials to craft their clothing) you will be friendly or hostile depending on what they look like. Again nothing is stopping you from acting how you want, it's just that by setting up the beginning in a certain way, players' behaviors are influenced organically. The rules don't even have to change, simply the circumstances.
God, I wish that were the case. The problem isn't in availability of resources, it's in the mindset of the players. Prior to the latest patch, resources were so abundant I could build a pretty sizable compound (4 foundations square, 4 stories tall and fully enclosed + metal doors with the beginnings of an outer fence) by myself in about an hour realtime. However, what usually happens is a guy who pulled a shotgun/pistol/mp5 out of a loot chest on his way to wherever, happens across me harvesting a rock next to my unfinished compound and shoots me dead on the spot, no preamble. Then I spawn (naked) and walk out my front door (or in one instance, onto my roof)... and he shoots me dead again with no preamble. The first time, I get angry because it's an inconvenience. The second time I log out because of the sheer stupidity of shooting a naked, unarmed man and wasting what in a real survival situation would be a near fortune worth of bullets. And when I say waste, I mean he emptied an M4 (which he pulled from my first corpse), then an MP5 (which he also got from my first corpse) worth of ammo at me as I was running away. He already had an inventory full of goodies from having just killed me. Complete rad armor, weapons, bullets, craft goods.... but he had to go the extra step and kill me for no gain on the respawn, and that is why everyone will always assume everyone else is aggro, and why the only cooperation will continue to be pre-arranged friends or "clans" that log in and work together from the outset, not the harmonious/spontaneous team building that would be so much more efficient in the long run. There is a significant population of players that can't get past the "i'm just doing what the game lets me, and the game lets me kill you and blow up your house" mentality from too many FPS shooters like CoD. possibly providing some sort of ally token, like a peace pipe or something, with a resource or crafting bonus for having ally's in proximity while doing stuff, or for the durability of your houses that increases to a point depending on how many players are sleeping inside of it. that doesn't break game mechanic, you can still raid and be a douchebag but now players have a real incentive to band together. I do like the idea of different color clothing depending on where you harvested the materials, and that would be pretty simple to code since there are already specifically defined resource areas.