I have the following XAML, that does all that it is supposed to, except that the MultiBinding on the FontSize fails on retrieving the Users. As you can see Users is an IEnumerable<UserData> that is part of the HierarchicalDataTemplate's DataContext.

How do I reference it??

<TreeView Name="AllGroups" ItemsSource="{Binding}" >
      <TreeView.Resources>
           <HierarchicalDataTemplate DataType="{x:Type PrivateMessengerUI:GroupContainer}"
                                     ItemsSource="{Binding Users}"
                                                  >
                  <Label Content="{Binding GroupName}"/>
            </HierarchicalDataTemplate>
            <DataTemplate DataType="{x:Type PrivateMessenger:UserData}">
               <TextBlock Text="{Binding Username}"
                          ToolTip="{StaticResource UserDataGroupBox}"
                          Name="GroupedUser"
                          MouseDown="GroupedUser_MouseDown">
                    <TextBlock.FontSize>
                         <MultiBinding Converter="{StaticResource LargeWhenIAmSelected}">
                               <Binding ElementName="Root" Path="SelectedUser"/>
                               <Binding RelativeSource="???"
                                        Path="DataContext.Users"/>
                         </MultiBinding>
                     </TextBlock.FontSize>
               </TextBlock>
           </DataTemplate>
         </TreeView.Resources>
      </TreeView>