List of available people

It would be helpful to be able to access a list of all active people. When one is working outside of normal business hours (night or weekend), and is needing some assistance from someone, it would be helpful to be able to pull up all members who are currently active. Even further, it would be helpful if the lists were categorized by department. Right now, the only way (that I'm aware of) to see people is if I know them by name and am able to search for them or I'm already in a space with them.