Just average the positions.
With two players:
camera.x = (player1.x + player2.x)/2
camera.y = (player1.y + player2.y)/2
With three players:
camera.x = (player1.x + player2.x + player3.x)/3
camera.y = (player1.y + player2.y + player3.y)/3
With four players:
camera.x = (player1.x + player2.x + player3.x + player4.x)/4
camera.y = (player1.y + player2.y + player3.y + player4.y)/4
...and so one. hopefully you see the pattern