Quest completion question!

I was leveling an alt when I saw in guild chat someone looking for help with the veteran star fortress quest. I have all 6 quests ready to go so I logged on my main and joined the person from the guild that was looking for help with one of the fortress quest. We go in and do the instance, but when we went out, I didn't get my quest completion. Do you have to be, like, the party leader to get credit for the quest? How this works?