Did you actually find players to complete the group missions with or are they soloable by now? I finished them when they launched and everyone was doing them, but getting a group together must be hell these days.
When I was leveling, the heroic missions were soloable, but I stopped doing them after awhile since I was constantly over leveled for the content. I don't know if they are soloable when their level cap is close to your characters level cap.
All the planetary heroics are super easy to solo and now that everything has level synch you'll never be more than a couple levels higher than the planet