Once your shifts have been added (using Add Shifts or Import) you can use AutoFill to automatically assign workers to achieve the best possible schedule for everyone in a matter of seconds.  AutoFill must be done one week at a time.

WhenToWork's AutoFill feature uses advanced mathematic algorithms to determine the best possible schedule for that week and automatically assign Employees to shifts based on granted time off, employee work time preferences, minimum time off between shifts, and any employee maximum shifts/hours/days you have set. (You also can choose to AutoFill based on Priority Groups or Seniority)

To AutoFill your Shifts 

On an UnPublished Weekly Schedule page 

When AutoFill is complete a window will appear that shows the new statistics for that AutoFilled week.

These stats can also be accessed at any time by changing the Schedule View to "Statistics."

You can see how the shifts compare to the most current employee work time preference and minimum time off between shifts etc. using the ANALYZE shifts function on the Sortable List view.

You can even Clear the AutoFilled assignments and AutoFill again to get slightly different assignments. Because AutoFill starts with a random ordering, if employee preferences or availability are similar our scheduling engine tries to randomize the assignments as much as possible. If repeated AutoFill does not give different results it is likely due to only a few employee preferences having been set, and where AutoFill is repeatedly trying to accommodate those preferences.

Note: If you have some set or "pinned" employees you can Import a saved Template that has all shifts unassigned except for those pinned employees and then you can AutoFill only the open shifts around that set of employees.