For WP8 Multi-Selection you'll need to use the Windows Phone Toolkit's LongListMultiSelector.
You can find code samples on how to use LongListMultiSelector here (and here for the code behind). Here are the relevant XAML code snippets:
<phone:PivotItem x:Name="BuddiesPivotItem" Header="Std longlistmultiselector">
<toolkit:LongListMultiSelector x:Name="buddies" Background="Transparent"
Margin="0,-8,0,0"
ItemsSource="{StaticResource buddies}"
LayoutMode="List"
IsGroupingEnabled="True"
HideEmptyGroups="True"
JumpListStyle="{StaticResource BuddiesJumpListStyle}"
GroupHeaderTemplate="{StaticResource BuddiesGroupHeaderTemplate}"
ItemTemplate="{StaticResource BuddiesItemTemplate}"
/>
</phone:PivotItem>
<phone:PivotItem x:Name="GridModeItem" Header="Grid mode">
<toolkit:LongListMultiSelector x:Name="GridSelector"
ItemsSource="{StaticResource PicturesAlbum}"
IsGroupingEnabled="False"
GridCellSize="210,180"
LayoutMode="Grid"
HideEmptyGroups="True"
ItemTemplate="{StaticResource PictureItemTemplate}"
IsSelectionEnabledChanged="OnGridSelectorIsSelectionEnabledChanged"
SelectionChanged="OnGridSelectorSelectionChanged"
/>
</phone:PivotItem>
When you run these code snippet you can see the following:
You can read more about the Windows Phone 8 Toolkit here.