They need to set their MDE for you to view their photos (and of course you can set it for them to view yours). And yes, you can set it so they can only view shared plans. They will see the photos with a watermark; you will see them without a watermark and can download them and share them via email, flash drive, etc. I believe that the only plans they could alter would be plans you all share, such as park pass reservations (and FP if it comes back by then). I don't think they could mess with your hotel reservations unless they logged into your MDE account, and only the person set as manager of an ADR can change it.