|XP is just a number|
Building on this, the PM needs to add time for
reading and understanding requirements
reading and understanding test cases and use cases
getting development environment set up, accessing and organizing your code repository for a group now
time spent dividing up coding, assigning and supervising coding
documenting the efforts of all these people
and the fun just carries on.
Do all the developers have the skills needed for the project or are they different levels of experience, different programming backgrounds?
A combination of time add ons and simple non emotional explanation of why this will increase time and costs is what you need to do. If you can train them into understanding that changing things in mid development is not a good idea and that resources and requirments need to be sorted out at the beginning of the project. The PM should grasp this if they are any good and help you present your case.