A simple little twist an old drill.

Start out with your basic 2v2 ground ball drill teaching all the fundamentals of a ground ball.

When a player wins the ground ball a third player from his line will immediately release.

The player picking up the ground ball must now roll away from pressure to give it to his team coming in.  Now you have a 3v2 to score on the goalie.

This works because players love to score, having the extra guy come in from your team gives the players that extra incentive to win the ground ball