There is more art than science to selecting the best core group members. The core group should be your project engineer(s), project controls manager(s), procurement manager(s), construction manager(s), and the project secretary/assistant.
Personal chemistry, gut feel, or a sense of ease must be part of the selection criteria. Focus on the candidate with the best training and experience, but if you don’t feel comfortable with them, move on to the next candidate.
Try to imagine how the candidate will relate to the other members of the core group. They have to work together. They have to like each other. You have to like them and they you.
If your original assessment is wrong, right that wrong as soon as possible. Don’t let disharmony or acrimony take root on your project. It is essential your team function as a team.
Corollary: Don’t hire a friend just because he/she is a friend. An incompetent team member is as destructive as an angry, hostile, disruptive teammate.
Get the best, but make sure they fit.