The content of the calendar in the Resource Planner can be set to either automatically update based on the available employees in the filter on the right hand side.
Next to the employee names there will be a checkbox if the Resource Planner is set to manual sorting.
To change this you go to Settings and find the option for "Always show content of employees list in the calendar" under the Other category.
The calendar arranges employees alphabetically by default but this can also be changed to manual in the same settings menu.
In manual mode it is possible to re-arrange the names in the calendar by dragging and dropping.