They should be told individually, not as a group. The risk of telling people ahead of time is that the company opens itself up to sabotage. For example, an employee continually shows up late for work. A co-worker can say something to the manager, then the manager must witness the behavior. High tech companies where employees are good enough in technical skills, but lack social interaction.
Social promotion is practiced.
Mixed Messenger Office politics. Wooden Stick Management and employees resist change. Escape Workalcoholics, rewarded for how much time is spent at the office. Shocker Good old employer. Stranger Afraid to do anything.