Try this:
<Setter Property="ItemTemplate">
<Setter.Value>
<DataTemplate>
<StackPanel Orientation="Horizontal">
<TextBlock Text="{Binding Name}" Margin="0,0,20,0" />
<TextBlock Text="{Binding MemberCount}" Margin="0,0,5,0" />
<TextBlock Text="Members" />
</StackPanel>
</DataTemplate>
</Setter.Value>
</Setter>