Start with 3 balls per group. 1 ball on the ground and played back and forth with the feet, while the other 2 are passed back and forth. Never hold the balls in hands for more than 1 second, always pass back immediately. Don't be nice to your partner, challenge them.
Then have 2 balls, with 1 player holding the ball in hand. Throw the ball up and while it's in the air, pass the other ball back and forth once. Make it harder by trying to pass 2, then 3, then 4, then 5 times while the ball is in the air.
2 balls per group - 1 player starts with both balls. Throw 1 high ball and during that time, quickly pass the other ball. The second ball should be caught before the high ball is caught. Add the challenge of passing the quick ball back.
3 balls per group - 1 player starts with all the balls. Throw them in an arc, one by one. All 3 balls should be in the air at the same time and then caught by the receiver.