If you are in a weak matrix organization then the functional heads may have more power than the project managers and may dictate who is allowed to be on your project team and for how long. These employees will still have to answer to their functional heads who may pull them off of a project entirely for whatever reason. If all parties are taking into account the best interest of the company then both the functional heads and the project managers will negotiate for their employee’s time resource so that their normal departmental work and their project work both get done. If an employee quits outright without giving two weeks or offering to train their replacement, then you will rely on the current project team members, completed documents, past meeting minutes, status meetings, and historical lessons learned documents to get the new employee up to speed.