Use Border to wrap RadioButton
<UserControl x:Class='SilverlightApplication3.MainPage'
xmlns='http://schemas.microsoft.com/winfx/2006/xaml/presentation'
xmlns:x='http://schemas.microsoft.com/winfx/2006/xaml'
xmlns:d='http://schemas.microsoft.com/expression/blend/2008'
xmlns:mc='http://schemas.openxmlformats.org/markup-compatibility/2006'
mc:Ignorable='d'
d:DesignWidth='640'
d:DesignHeight='480'>
<Grid x:Name="LayoutRoot" Background="White">
<StackPanel>
<Border Margin="5" Padding="5" BorderBrush="Yellow" BorderThickness="1"
CornerRadius="5">
<StackPanel>
<RadioButton Content="Group 1"></RadioButton>
<RadioButton Content="Group 1"></RadioButton>
<RadioButton Content="Group 1"></RadioButton>
<RadioButton GroupName="Group2" Content="Group 2"></RadioButton>
</StackPanel>
</Border>
<Border Margin="5" Padding="5" BorderBrush="Yellow" BorderThickness="1"
CornerRadius="5">
<StackPanel>
<RadioButton Content="Group 3"></RadioButton>
<RadioButton Content="Group 3"></RadioButton>
<RadioButton Content="Group 3"></RadioButton>
<RadioButton GroupName="Group2" Content="Group 2"></RadioButton>
</StackPanel>
</Border>
</StackPanel>
</Grid>
</UserControl>
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