At each reporting date you need to make an estimate of the number of employees you believe there will be still employed by the vesting date. There are several ways in which the examiner can test this but I think the one you are referring to is when you adjust the number of employees by those who have already left, as they will not be entitled to the share based payments, and also adjust for those who you expect to leave in the future, as again they will not be entitled to the share based payments if we expect them to leave.
You need to work through all the question on IFRS 2 to get good at identifying how to calculate the expected number of employees.