Interesting, if you telling the truth, yet on the previous H4, All The Pieces/Dark Design, you have to have people exactly at that point... That I know as a FACT!
So, even if you can do Last Stand, you can't do All The Pieces/Dark Design... They will get to the platform and told " You are not eligible for this conversation". Oh, the other 2 guys that we joined up with, also tried to get their guild mates to help, they had the same result... If you know a trick I do not, please let us know.
I think it is buggy or has been patched.
When I first tried to do "All The Pieces" on my main (2 weeks ago), I asked various guildies for help. Some had fully completed the quest chain; some hadn't started. All of them got the "not eligible" message at the platform. I then completed it after waiting for another guildies to get up to the same step.
Now on my alt, I was able to solo completed "All The Pieces" (yes a dps merc with mako can kite, heal and interrupt). Yesterday I met someone at Ilum to finish the seeker droid quest; he happened to be at the "All The Pieces" step; so I offered to help (hoping to find 2 more people to do Shroud's Last Stand after). To my surprise, I was able to get pass the platform. I got the "not eligible" message when I clicked; but then I got the "spectator mode" cut scene.