In essence, teamwork is an emergent property of a set of players playing well, and the important point to note here is, emergent property .... that is so critical when trying to unravel the mystery of teamwork.
Above is a proto-definition I suppose you could say, I realise it's my definition and there may be others out there but this one is specific to paintball and is as accurate as one can get.
I also realize that if I'm right, it suggests it's maybe out of our reach in terms of trying to train 'teamwork' directly.
Actually, it isn't like this in practice at all because even though the amorphous quality of teamwork is not directly trainable (because it is an emergent property of something else) we can direct the focus of our training toward whatever precursor activity is creating it, and in this case, it's players, playing well.
Now this in itself is maybe a hard concept to grasp but at least we have the fundamentals at hand to break down the idea of playing well.
Playing well is a combination of upholding whatever techniques are deemed appropriate, fast and accurate snap shooting etc, as well as drawing on whatever experience you have accumulated.
When each player on the field is deemed to do this, the sum of the team then becomes greater than its component parts and it is this we now interpret as teamwork .... that is a working definition ... well, it's my definition anyway.