How to Share some project parts with another google account (user)

I ve just installed Appflowy. I want to know followings:

  1. I just double click .deb file of AppFlowy and installed it on MX Linux. Do I need to install Flutter etc ?
  2. Im unable to register with Google account. Why?
  3. I want to Share some parts of my project with some other google account), how to do that ?