Task Groups are just collections of Units - if you locate a TG at a planet there is actually no indication of whether that TG is on the surface or in orbit (I believe that this is for reasons of ease of coding rather than any deliberate decision). I just create seperate TGs for each type usually with names like <Planet Name> - OrbDefGrp and <Planet Name> - SurDefGrp.
This does make it impossible to differentiate alien Orbital Defence Bases from PDCs as I discovered when my fleet inadvertantly plastered a planet so that it now has -13% production due to Radiation/Dust effects - I'd brought a serious invasion force with the intention of investing the PDCs with ground troops rather than turning them into strontium-90 sunset material. Ah well, such is life!