Welcome To The @ Squad

1) The entry fragger(s)
The entry fragger is one of the most important players in the team. This player has a high tendency for flick shots, excellent game sense for checking corners, and most definitely a rifler. This is your point man, he's the first one in, and he's the last one out. This player doesn't need to be your smartest player, or the best at calling out positions, he is pure skill.
Some teams have one, others have two. Don't be mistaken. This might be your best player, but I have played on teams with 5 top fraggers and they almost never work out.
2) The support player(s)
These are your information gatherers, your bomb planters, and your kit buyers. These are your set-up professionals. They set up a bombsite for entry with smokes and flashes and need to learn to do as such. They are going to give you the best information, and ideally are the ones to suggest the best next move for the team. Because of this, I think it works a lot better to have an "alternative" caller on a team that doubles as this role. This way, if the caller is dead the team is not lost and the caller does not have to micromanage from a death cam.
These guys may not be your top fraggers, and they may choke on a clutch, but these are the players that allow you to win round by round as opposed to amazing plays.
3) The AWPer/Auto-sniper
This one is pretty straight forward. In CS:GO a sniper is not always a necessity, but can enhance game play in many situations. You only need one of these at most. Just because an AWP is on the ground doesn't mean you have to pick it up.
This player is smart, cautious, and a hell of a shot. Unlike the entry fragger you aren't looking for this guy to get 5 amazing kills. You want him to get smart picks without getting killed. He's the tide turner. Round by round your awper is either going to put your team at an advantage or disadvantage depending on which way the kills go.
A round 5v4 is already 70% in your favor. A round 5v3 is 80-90% in your favor. The AWPer is not greedy. He gets that pick and falls back without dying preferably. I suggest pairing an AWPer with either a support player or....
4) The clutch player
This is going to be the most debatable position. The above are all tried and true, but many people don't believe in the term clutch player. We all have clutches, and we all fail sometimes, but I am mentioning this player because I think he overlaps qualities with all of the above.
The clutch player has immense game sense. The clutch player could be your AWPer, a support player, or even the caller. He thrives on pressure and adrenaline. The clutch player may or may not be your top fragger/entry fragger.
Like the AWPer the clutch player is not greedy. He plays smart, he baits the bomb, he peeks when he has to. Game sense, game sense, game sense. Often the caller will have the best game sense in the game which is why I think he could very well be the clutch player, but again, the caller has a lot more on his mind than making a shot.
TL;DR: In-game leader, two support/solid players, AWPer, the top/entry fragger, and the clutch player.
5) The caller: Pretty self-explanatory. Callers go as far as spending hours a week in researching to simply calling on the fly. The caller can be as involved as he wants to be.