Not sure what you mean as I've never had that issue. Did they enable the ability to allow people even of the same class into their instance?
I think you could, but only one character's story at a time gets instanced.
Basically, in our case, if I was a jedi knight, I could invite other people into my party, which would allow them into instances specific to my characters storyline. Usually, they'd get to spectate conversations, so they could follow my story, but there's times when they're not able to see what I'm seeing, and so can't follow the entire story.
The game breaking part came when, even in my party, they simply weren't going into my instance at all, and vice versa.