Now, let's say you have to do some analysis on layers inside the group layers, you can loop the layers as follows:
mxdPath = r"c:\temp\mapDoc.mxd"
mxd = arcpy.mapping.MapDocument(mxdPath)
layers = arcpy.mapping.ListLayers()
for layer in layers:
for subLayer in layer:
print "This layer is in a group layer: " + str(subLayer.name)
This will print a list of layer contained in a groupLayer.