Since the cube is a Sean's place that is useless and even if he is a rebel I don't think that another yeerk would go over well with Jo.
NOTE: The morphing cube is in an undisclosed location, though the other characters should assume it's at Sean's place.
Also, for the sake of story plot convenience, let's say that Sean, Kristen, and Chad arrive at the barn now.

That's where they're going, and they wouldn't be able to see Brian, Edward, and Olivia out in the woods from the barn.