How can I filter just underutilized resources on the Utilization tab?
Use the Diagnostic Filter>Underutilization Filter menu command.
Set the Underutilization criteria on the Diagnostic Settings tab in the Tools>Options dialog.
If your Get Data includes many periods with no demand, set the # of Periods to an appropriately large number.
If it includes just the current planning horizon where there should be demand for everyone, set the # of Periods to 1 (or another small number) to get every resource with at least 1 period of underutilization.