You are confusing mojoPortal users with Windows users.
mojoPortal has no control over file system permissions, that is a feature of Windows and applies to Windows users.
The ApplicationPool runs as a Windows user and it is the same no matter who is visiting the web site no matter if the user is anonymous or signed into the site as a user with admins role. So you cannot really manage windows file system permissions per mojoPortal user account since from Windows point of view it is always the same Windows user that the app pool and web site code is running as.
The Windows user that is the identity on the applicaiton pool must have read permission for all the web site and must have full control on /Data and /App_Data folders for mojoPortal to work correctly.
In older verisons of Windows and IIS the Network Service user was often used as the user that is the identity of the application pool, and you could assign that user as the app pool identity if you wanted to make things a little simpler.
In newer verisons of IIS they introduced virtual users and the ApplicationPoolIdentity, you can learn more about that from this article and some googling.
Hope that helps,