In the staff-planning overview you will find projects and events with various columns that show you date, time, location, customers, registered employees, number of employees in the team and the name of the planner

In the "Applied" column where the "registered" employees are located, you will also see numbers in green or in red. These numbers describe people who registered or de-registered since the last time a planner opened the staff-planing for this project. 

These notifications look like the following and appear in the "Applied" column next to the black number of registered employees:

In the screenshot below the entire process of registering/scheduling an employee is illustrated again.