Before starting any collaborative PBL approach in a classroom, I think it is necessary that team members sign a team work contract.
Why? Because in real life being able to overcome a challenge involves commitment, and so does in class.
That's why when we started PBL work at #ictclil_urjc, the first step was providing the team with a contract to sign, as a way to make them aware of the importance of their projects, the respect the other team members deserve, the responsibility and conscientious work such an approach entails.
I hope you find it useful and interesting for your own lessons.
Feel free to download it and adapt it to your own classroom needs.