Who said they weren't in guild ?
It's a heroic that's only available at a specific stage of that questline and not repeatable. Without the help of group finder no matter how big your guild is, the chances to get a group are really low.
You are forgetting something. Our guildie was at a final stage for macrobinoculars. He asked for help, and three of us stepped in, even though we did NOT have the quest at that stage. (I didn't even pick up the binoculars on that particular character). As soon as the person who has the quest steps into the instance, any other 3 can join him. It's actually really easy to get this quest done if you have friends.