Charles,
Actually the program shouldn't notify all the users if this was not set on purpose.
By default the program send you the notification if:
1. Somebody made any change in the task you are assinged to do (you are assigned Resource)
2. Somebody made any change in the task you created (you are the Owner of the task)
By your wish (if you are neither owner nor assigned resource) the program can send you the notification if you are supervisor of:
3. A certain resource (Edit Resource window, Watch List tab)
4. A certain task group (Edit Task Group window, Watch List tab)
5. A certain task (Edit Task window, Watch List tab)
If you receive Notifications of tasks you are not assigned to or you didn't create, please, make sure you do not supervise these tasks in any way (#3,4,5). Mind, that if you supervise any Task Group (#4) you automatically supervise all its subgroups no matter if you are checked on Watch List tab of subgroups or not.