I received the invitation as well. Took the assessment yesterday and there really is no way to prepare for the responses, it is a fair tool to monitor your work personality and soft skills.
I also think that as a pilot program, they are sampling a random assortment of applicants, so I would not take it necessarily as an indicator of admission or not. In fact, I would think they would want a random sample from folks they do and do not admit to make it a better tool to help in admissions decisions.
Hope everyone is surviving their last short period of the unknown, we are in the home stretch before decisions are released. Good luck to all!