I have a 2D top-down engine, which uses the A* algorithm for pathfinding. I want the multiple sprites on screen to formate with each other before advancing n the players position. At the moment each sprite calculats the nearest friendly and the cost of moving in said friendly sprites direction is decremented. When several sprites are grouped together they then move towards the player. This method is quite processor heavy, and not exactly fool proof. Any sugestions on how to make sprites formate well would be greatly appreciated.

This paper may provide some inspiration. Using reactive techniques for the enemies, you can achieve a variety of different formations at minimal computational cost. Leader referenced formations, for example, will alow you to plan a path for one enemy and have the others follow in a nice formation without having to plan paths for all of them.

http://www.cc.gatech.edu/ai/robot-lab/online-publications/formjour.pdf