Ok. so I'm getting some cool results now..
I've added an initial impulse of -10 on the Z plane to all of the players.
I did a little hack in the render function that follows one of the players on the Z plane and keeps them in sight.
But I think we need something smarter.

I think we need 2 things:

1. on the gravity calculation function, always add a force which draws the character towards the center of the screen. Sort of like the sun.
2. Each frame, calculate the average Z of the players and place both the camera and the starfield in (0, 0, AverageZ).

That (0,0,AverageZ) should probably also be used for the pulling into the center of the screen.

I think that should work quite nicely!