When N different users are "added" to a certain project or
subproject, whenever you are logged in as one of these users and select
the "All Projects" scroll-down menu in the upper right corner of Mantis,
you see N instances of each sub project...
Suppose I have a project called "Project 1" and a sub project called
"Sub Project 1".
I will add user foo to "Sub Project 1" (I.E as a reporter).
I will add user bar to "Sub Project 1" (I.E as a reporter).

I am logged in as user foo and when I'll click on the scroll-down box I
will see:

Project1

Sub Project 1
Sub Project 1