Whenever a value is selected from a combo box the following `SelectedIndexChanged` event triggers. So, you should try something like this:
Private Sub ComboBox1_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles ComboBox1.SelectedIndexChanged
' TODO : Here you can also set index based criteria
' If (comboBox1.SelectedIndex = 2) Then
' groupBox1.Enabled = True
' End If
groupBox1.Enabled = True
End Sub
Visit MSDN documentation for `ComboBox` control as well.
Hope it helps!