How well do you know your teammates? Do you know their first names? Would you feel comfortable lending one of them $20? How well do you get along with them?
Teamwork is what separates the CAL-i from the CAL-m, the pug team from the new CAL-o superstars. Most teams in source have crappy teamwork, relying instead on their aim to carry them through a match. But aim can only carry you so far, and teamwork is something that will allow you to win matches against more talented foes.
If you don't like someone, you can't play with them. If someone on your team has an amazing shot, but they don't get along with anyone, and they don't communicate, then they are hurting your team. It is better to cut them and focus on your teamwork.
In this vein, don't play the blame game when the round is over. You should and you have to talk about what went wrong, but don't fight about it. If someone says something about how you played, do not get defensive. I know a lot of people have big egos in this game - you have to set them aside. If the guy ivy keeps dying because you aren't covering mid in a proper manner, then listen to him. If you were slow on a rush, understand why and fix it. What you should not be doing is arguing about it. Arguing and blaming builds resentment, and resentment leads to your teammates wanting you to do poorly, which obviously leads to poor teamwork.
In order to have teamwork, you must have trust. If you find yourself telling your teammates how to play over vent while watching them play, then you don't trust them. If you can't trust your teammates to play the game, you probably won't be trusting them to watch their spot, and if you can't trust them to hold their spot, you won't be able to hold yours. Trust your teammates, and if they do something stupid, tell them when they are dead. It will stick in their mind better if they lost the round because of what they did, and it will allow them to concentrate better if you aren't talking while you are dead.
About talking while you are dead - to build teamwork quickly, stop it. If possible, use ingame. This forces you to stop calling when you are dead, and makes you talk while you are alive. The ability to calmly call out where your opponents are while shooting can be a difficult one, but it will help your team immensely. When reaction shots are measured in fractions of a second, telling your teammates where the enemies are 2 seconds earlier might save their (ingame) life - or yours.
What should you be talking about? Location, location, location. If you are at the point where you know all of your teammates voices, and you know where they all play on every map - and how they play it, then everyone can just call out what they see. Here is an example from a dust2 scrim.
Long: Flashbang. Flashbang. Smoke. 1 guy dropped pit
Cat: Flashbang cat. 1 guy cat, dead.
Long: Another guy long, working their way up
Cat: I pushed up cat, mid guy look long
B: I'll rotate to long, you stay b
Long: I don't see anymore long, fall back to b
B: Oh ****, flashes B, I can't see
B: I'm at door, peeking in
You get the point? Call flashes, call nades, call when you see people. If you rotate or leave your spot, let someone know. If you see the bomb, let people know. On t side, don't enter 'ct territory' (any part of the map where CT's traditionally play) without letting someone know and finding out if you have a teammate hitting a close area at the same time (such as long and cat). So many times have I seen 3 guys go long a, try to cross, and die from mid/cat, then 2 more guys mosey up cat and get picked off because the other team can focus entirely on them. Split their focus! Work as a team.
So, in conclusion: Be friends with your teammates, don't argue and fight about who's fault it is the round was lost, focus on talking while you are alive, and keep your teammates informed about what is going on at your location. I know a lot of this stuff isn't revolutionary but it will help