I have determine that my AutoCad 2009 will allow me to drag/drop layers to an existing Group Layer Filter, but it will not allow me to right click on the Group Filter>Select Layers>Add. Once I go to the model space an select layers then "enter" no layers are added to the Filter Group and it gives me an error message that says "unable to modify layers while another command is active". Anyone got any ideas on what the problem could be?