Whether they are implicitly or explicitly defined the requirements for the system need to be familiar to the people participating in the group.
The ideal group size for a CRC card session is five or six people. This size generally allows everyone to productively participate. In groups of larger size productivity is cut by more disagreements and the amount of participation by each is lower. If there are more than six people, one solution is to have the extra people be present strictly as observers.
The group five or six people in the core group should be composed of developers, domain experts, and an object-oriented technology facilitator.