<Grid>
    <TreeView Name="departmentsTree" ItemsSource="{Binding DepartmentsTopLevel}" HorizontalAlignment="Left" Width="232">
        <TreeView.ItemTemplate>
            <HierarchicalDataTemplate ItemsSource="{Binding Childrens}">
                <TextBlock Text="{Binding Title}"/>
            </HierarchicalDataTemplate>
        </TreeView.ItemTemplate>
        <TreeView.ItemContainerStyle>
            <Style>
                <Setter Property="TreeViewItem.IsExpanded" Value="True"></Setter>
            </Style>
        </TreeView.ItemContainerStyle>
    </TreeView>
    <DataGrid ItemsSource="{Binding Path=Employees}" AutoGenerateColumns="False" CanUserAddRows="False" Margin="237,0,0,0">
        <DataGrid.Columns>
            <DataGridTextColumn IsReadOnly="True" Header="Фамилия" Binding="{Binding LastName}" Width="100" />
            <DataGridTextColumn IsReadOnly="True" Header="Имя" Binding="{Binding FirstName}" Width="150" />
            <DataGridTextColumn IsReadOnly="True" Header="Дата рождения" Binding="{Binding Birthday, StringFormat=\{0:dd.MM.yyyy\}}" Width="100">
                <DataGridTextColumn.CellStyle>
                    <Style>
                        <Setter Property="TextBlock.TextAlignment" Value="Center"/>
                    </Style>
                </DataGridTextColumn.CellStyle>
            </DataGridTextColumn>
        </DataGrid.Columns>
    </DataGrid>
</Grid>

public class OrganizationViewModel : ViewModelBase
{
    public IDataContext Context;
    public IWindowService Service;

    public OrganizationViewModel()
    {
        Context = new DataContext(); // todo IoC resolve
        Service = new WindowService(); // todo IoC resolve
    }
       
    public IEnumerable<Department> DepartmentsTopLevel
    {
        get
        {
            return Context.GetDepartments().Where(d => d.Id == 0);
        }
    }

    private Department _currentDepartment;
    public Department CurrentDepartment
    {
        get
        {
            return _currentDepartment;
        }
        set
        {
            _currentDepartment = value;
            OnPropertyChanged(() => CurrentDepartment);
        }
    }

    private ObservableCollection<Employee> _employees;
    public ObservableCollection<Employee> Employees
    {
        get
        {
            return _employees;
        }
        set
        {
            _employees = value;
            OnPropertyChanged(() => Employees);
        }
    }
}

public void OpenOrganizationWindow()
{
    var view = new OrganizationWindow();
    view.Owner = ActiveWindow;
    var vm = new OrganizationViewModel();
    view.DataContext = vm;
    var tree = (TreeView)view.FindName("departmentsTree");            
           
    tree.SelectedItemChanged += delegate(object sender, RoutedPropertyChangedEventArgs<object> e)
    {                                
        vm.CurrentDepartment = e.NewValue as Department;
        var employees = vm.Context.GetEmployees().Where(d => d.Department.Id == vm.CurrentDepartment.Id);
        vm.Employees = new ObservableCollection<Employee>(employees);
    };

    view.Show();
    view.Closed += (a, b) => { Application.Current.MainWindow.Focus(); };
}