I have never worked at Joes, but being a bartender and server for 15 years I can say that it does not matter if the table has separate checks or not. The number of people at the table is what qualifies the gratuity. Even if everybody at the table would have paid individually, the grat would have been added to each check.

Menus normally read "Gratuity will be added to parties of 8 or more"

Not "Gratuity will be added to checks of 8 or more"