Our Sienna has captain's chairs in the front and second rows, and a split bench in the third row making it a seven seater. I assume you have to have a second row bench seat to make it an eight seater.
In the seven-seat configuration, you could seat two in the back row and fold the remaining section of the bench flat giving you some additional cargo space. You probably could cram a large suitcase for each passenger and a few assorted smaller pieces in there if you do that, but I can't guarantee anything. If everyone has a couple of suitcases, however, forget it.