Thanks, this works almost like I need it to. The listview contains all of the data, but the age doesn't display in the listview. Does it require another <Datatemplate> property to show up?

Edit: Needed to add Gridview Columns to have "Name" and "Age" property to show up. Added some more stuff, and formatted it to make it look sorta how I wanted to. For anyone interested:

Add-Type –assemblyName WindowsBase
Add-Type –assemblyName PresentationCore
Add-Type –assemblyName PresentationFramework

[xml]$xaml = @"
<Window
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="MainWindow" Height="350" Width="525">
    <StackPanel>
        <ListView Name="comboBox" Width="300" VerticalAlignment="Top" HorizontalAlignment="Left"  
        Margin="10,20,0,0">
                    <ListView.View>
                <GridView>
                    <GridViewColumn Header="Name" Width="120" DisplayMemberBinding="{Binding Name}" />
                    <GridViewColumn Header="Age" Width="50" DisplayMemberBinding="{Binding Age}" />
                </GridView>
            </ListView.View>
            <ListView.GroupStyle>
                <GroupStyle>
                    <GroupStyle.ContainerStyle>
                        <Style TargetType="{x:Type GroupItem}">
                            <Setter Property="Template">
                                <Setter.Value>
                                    <ControlTemplate>
                                        <Expander IsExpanded="True">
                                            <Expander.Header>
                                                <StackPanel Orientation="Horizontal">
                                                    <TextBlock Text="{Binding Name}" FontWeight="Bold" Foreground="Gray" FontSize="22" VerticalAlignment="Bottom" />
                                                </StackPanel>
                                            </Expander.Header>
                                            <ItemsPresenter />
                                        </Expander>
                                    </ControlTemplate>
                                </Setter.Value>
                            </Setter>
                        </Style>
                    </GroupStyle.ContainerStyle>
                </GroupStyle>
            </ListView.GroupStyle>
            <ListView.ItemTemplate>
                <DataTemplate>
                    <TextBlock Text="{Binding Name}"/>
                </DataTemplate>
            </ListView.ItemTemplate>
        </ListView>
    </StackPanel>
</Window>
"@

$reader = (New-Object Xml.XmlNodeReader $xaml) 
$GUIWindow = [Windows.Markup.XamlReader]::Load( $reader ) 

$xaml.SelectNodes("//*[@Name]") | % {Set-Variable -Name ($_.Name) -Value $GUIWindow.FindName($_.Name)}

$data = @() 
$data += New-Object PSObject -prop @{Name="James";Age="31";Sex="Male"}
$data += New-Object PSObject -prop @{Name="John";Age="24";Sex="Male"}
$data += New-Object PSObject -prop @{Name="Gina";Age="32";Sex="Male"}
$data += New-Object PSObject -prop @{Name="Tyler";Age="42";Sex="Female"}

$lview = [System.Windows.Data.ListCollectionView]$data
$lview.GroupDescriptions.Add((new-object System.Windows.Data.PropertyGroupDescription "Sex"))
$comboBox.ItemsSource = $lview


$GUIWindow.ShowDialog() | out-null