There are some co-op aspects to it.
All gear that you find can be shared. I have at times traded some painkillers or a bottle of water for a warmer jacket or traded a AR for a Shotgun if I already an AR that I liked. You can also help each other in combat or revive a fallen agent if you feel inclined.
You can even bring your group in as well and you will all awaken in the same safe room. This is where it gets tricky. If you travel as a big group you need to know that items such as tools, weapon parts, and electronic parts cannot be shared. So if I see a crate and open it and get said parts and take them then the crate is empty for everyone on the server. Sometimes this is a good way to track other agents paths too if you start finding a string of empty crates then you know somebody has been through the area.
So eventually you'll have to break away from the group in order to not slow anybody else down as your infection timer is ticking. It's not uncommon for a solo runner to make it to the Dark Zone but it is rare, at least in my experience, for a group to make it that far and still be together.
The real trick is managing this in a PvP situation. I've come across fellow agent who I've helped just to have them shoot me dead 30 seconds later so they can take my gear. I'd be lying though if I said I haven't done the same thing. In PvP if you come across a downed agent you have the option to revive or kill. I usually revive them and for the most part they just say thanks and take off running. But knowing that the closer you get the Dark Zone the likely hood of them having a lot of gear and weapons is high it makes it real hard not to kill them and take their stash.
As far as end game rewards go you get stuff based on the following.
How long you lasted
How far you progressed ie: Did you build your filters, reach the Dark Zone, find the antiviral, craft your flare gun for extraction, call in extraction, what kind of gear you had on you when you died, did you find weapon caches to extract etc etc.
If you do extract you get to keep any caches you found along the way as well as the normal high end gear you are rewarded for finishing. The caches are sealed until post game so it's a mystery as to what you get.
The game is a constant balancing act which makes it great.
I've had some games where I start off finding a ton of great gear and weapons and then get over confident and end up lasting only 10mins. I've had other game where I don't find anything good until 45mins in and have had to sneak around the whole time. You have to be able to adapt to the situation and mold your gameplay based on what the map gives you.