async void GroupTappedAsync(object sender, ItemTappedEventArgs e)
 {
     var item     = e.Item as Group;
     var editPage = new GroupPage(new GroupViewModel(item, true));
     await Navigation.PushAsync(editPage);
 }
 async void ExistingParticipateTappedAsync(object sender, ItemTappedEventArgs e)
 {
     var item     = e.Item as Group;
     var editPage = new GroupPage(new GroupViewModel(item));
     await Navigation.PushAsync(editPage);
 }