I think that Resource Management took the place of this functionality. With Resource Management you create a Resource Plan which is attached to the Demand, Project, or Project Task record (or any record that extends off the Planned Task table). Instead of assigning multiple assignees you allocate a certain amount of time to multiple resources, then you get another record called a Resource allocation record that gives the time that each resource was allocated to a given task/project. This enhancement allowed for better resource reporting which is the real win with the resource management app outside of the actual scheduling aspect giving resources the ability to show their own availability.
I am not sure if it is possible to use the User resource table unless you uninstall the current Project Management plugin. The table doesn't even exist for me in a Eureka or Fuji instance. Check with your account rep on that but I would try the Resource Management app first since you have it and may give you better reporting and better interaction with resources.
