If I add more than 5 employees and some leave, will I still be charged for the leavers?

No.

The fees for additional employees are based on the number of staff that were employed during a one month period. If you had 10 employees during the first period but only 9 during the second period, the fees would reduce accordingly.

If one employee leaves and another joins the following day, typically both would be counted as having been employed during that period.