We have two PEOPLE from our team who hardly participate in anything
Persons might even be correct, it's just very unusual.
The billing system/invoicing process is not operating correctly
My toddler would never stop crying for the whole night
My toddler would not stop crying for the whole night
My toddler never stopped crying for the whole night
Most of the parents let their baby cry at bedtime
If you are referring to something that is generally done, a better sentence would be:
Most parents let their babies cry at bedtime
Your sentence is actually close to ok, but only after describing a specific group of parents and a specific time. That would be unusual but possible. For example, at the family camp in July, most of the parents let their babies cry at bedtime. The most of the parents part is what makes it refer to a specific group of parents.