When you will create a new project with new co-workers and if you have checked notify co-workers then the co-worker will be notified. After creating the project when you will edit the project with new co-worker you have to again checked the notify co-worker option. This is the default system.
The system is- When you will add new co-workers in the middle of the project then if you want to notify them then you have to check the notify user option and if you do not want to notify them then leave it as uncheck. It will not set automatically all the time. It is our default system.