Scrum defines three distinct roles that must work together as a Scrum team to produce a successful product. The product owner is responsible for understanding what stakeholders require, interpreting these requirements for the team, and providing clear prioritized direction. The team is composed of those people actively engaged in building the product. The team collectively and members individually are responsible for building the product according to the priorities and requirements provided by the product owner. The team is completely responsible for deciding how to achieve its goals and how to organize its work. Finally, the ScrumMaster is responsible for the health of the process, reporting the progress of the team, and removing impediments to progress.
With these roles come a number of promises between the three parties, the Product Owner, the ScrumMaster and the Team. These promises should be formalised so that everyone knows where they stand.
Contract between the Organization and the Team
Sample Contract between Members of the Scrum Team
You will probably need to customise these document, but they are a good start.