<TreeView ItemsSource="{Binding Groups}" Name="tvGroups" AllowDrop="True"
          Drop="tvDrop" DragOver="tvDragOver">
          <TreeView.ItemTemplate>
                    <HierarchicalDataTemplate ItemsSource="{Binding Participants}">
                          <StackPanel Orientation="Horizontal">
                               <TextBlock Text="{Binding Name}" />
                               <Button Tag="{Binding .}" Click="Button_Click_2">
                                    <Image Source="Resources/cross.png" />
                                </Button>
                          </StackPanel>
                          <HierarchicalDataTemplate.ItemTemplate>
                              <DataTemplate>
                                  <StackPanel Orientation="Horizontal" >
                                       <TextBlock Text="{Binding Alias}" />
                                       <Button Tag="{Binding .}" Name="btnDeleteParticipants" Click="btnParticipants_Click" >
                                            <Image Source="Resources/cross.png" />
                                       </Button>
                                  </StackPanel>
                              </DataTemplate>
                          </HierarchicalDataTemplate.ItemTemplate>
                    </HierarchicalDataTemplate>
          </TreeView.ItemTemplate>
</TreeView>

private void tvDrop(object sender, DragEventArgs e)
        {
            if (e.Effects == DragDropEffects.Copy &&
                e.Data.GetDataPresent(typeof(Participant)))
            {
                Participant data = e.Data.GetData(typeof(Participant)) as Participant;

            }
        }