It is absolutely necessary for PBL to have collaboration in teams. At the beginning students cooperate (if that) and think they collaborate.
Now, over the years I've done a lot of teacher trainings as a trainee and as a trainer and it was always useful to play an ice-breaking games at the beginning. One of the games that brought teams in collaborative mood was the Balloon Tunel game.
BALLOON TUNNEL BALL
This is exactly the same as the usual tunnel ball game except you use a balloon instead of a ball. Everyone is divided into teams of 6-8 and must pass the balloon between their legs as they stand/bend in a straight line. The person at the back of the line receives the balloon and runs to the front to start the process over again. The first team to go through all its members without bursting the balloon is the winner.
picture taken from diypartytips.net