My name is Tim Zook, I'm the lead developer for MyBookTable/MyBookProgress. Thanks for your suggestions, implementing multi-user support for MyBookProgress is something that would be great to add in the future.
Unfortunately we are quite occupied with other projects at the moment, which means that we won't be able to look into this right away. Thanks for reaching out to us though, we'll let you know if this is something we're able to move forward with in the coming months.