没有找到相关方法，但是有对图层组进行操作的资料。
https://gis.stackexchange.com/questions/43620/how-do-i-reach-a-layer-inside-a-grouplayer
http://blog.csdn.net/comeonyangzi/article/details/26142391
http://bbs.csdn.net/topics/390915561#new_post
https://saeidp.wordpress.com/2010/11/18/arcobjects-working-with-group-layer/
http://help.arcgis.com/en/sdk/10.0/arcobjects_net/componenthelp/index.html#//0012000000vz000000