You can checkout a key to use the salon when it’s closed, but the following rules apply.

Stylists can do each other’s hair when the salon is closed, but they must pay the backbar charge with a credit card on a ticket in Mangomint.
Stylists cannot do paying clients when the salon is closed. You cannot take money for anything done inside the salon when it is closed without prior approval of the salon. (see the exception below for models)
Stylists must get owner approval to do models when the salon is closed. Models must pay the backbar charge with a credit card on a ticket in Mangomint.
Employees who check out a key must notify an owner of when they will be in the salon.