Figuring out the bill after a group dinner is tough enough without having to cover those who left early and accidentally miscalculated how much they owed. Settle the dinner bill when they leave to avoid having to awkwardly ask them for more money later or overpaying at the end of the night to cover them. Image from Guilhem Vellut.
I've had this happen before, especially when those who need to leave early pay cash rather than using a service like PayPal where you can "charge" someone an exact amount. Even with a service like that, you end up covering someone until they pay you back.
Of course, most people don't mean to stiff you on their portion of the bill, but forgetting to add in for that share plate or just not adding enough is an easy mistake to make. Rather than discovering this later, just ask your waiter to settle the bill when your friend has to leave. Anyone who stays to keep hanging out can open another tab for drinks.