So question.. I joined up with my BF and we ran around today and unlocked all the instances to do Invaded District Union Arena.
He's on WT5, I am on WT3.
His brother joined us on the 3rd/final instance, and then the three of us all did Invaded DUA.
BF got it completed, I did not, nor did his brother.

Why is that? We had thought that since it was currently invaded for Rook, it would have completed also for his bro and myself.

Does this need to be done in our own instances?

Invaded DUA is available to Rook (until he did it) and his brother on their WT.
It is not available for me on mine, could that be why?