There is no command to move to a specific zorderposition

In your particular case you could do it in 2 steps 

Sub moveIT()
Dim osld As Slide
'change to actual slide number
Set osld = ActivePresentation.Slides(1)
osld.Shapes("Group1").ZOrder (msoBringToFront)
osld.Shapes("Group1").ZOrder (msoSendBackward)
End Sub
More generally you would have to use a loop

Sub moveIT2()
Dim osld As Slide
Set osld = ActivePresentation.Slides(1)
Do
osld.Shapes("Group1").ZOrder (msoBringForward)
Loop While osld.Shapes("Group1").ZOrderPosition < osld.Shapes.Count - 1
End Sub