Put users into mantis with global level:
A - manager
B - manager
C - manager
D - manager

X - developer
Y - developer
Z - developer

Create project and put users with level
A - manager
B - reporter
X - developer
Y - developer
Z - developer

now call project_get_all_user_rows( project_id, MANAGER )
According to function 1st you select all users with global MANAGER level:

SELECT id, username, realname, access_level
FROM mantis_user_table WHERE enabled = 1 AND access_level >= MANAGER

and put results into $t_users array

then you search all users assigned to the project:

SELECT u.id, u.username, u.realname, l.access_level
FROM mantis_project_user_list_table l, mantis_user_table u
WHERE l.user_id = u.id
AND u.enabled = 1 AND l.project_id = 5

then you unsetting from t_users elements that didnt meet criteria on 2nd list.

Global Users:

A - manager
B - manager
C - manager
D - manager

Local Users:
A - manager
B - reporter
X - developer
Y - developer
Z - developer

On both lists there is A and B user so X,Y,Z wont appear on output.
A on both lists meets MANAGER level so it will stay on output list.
B is REPORTER on 2nd list and will be unset.
C and D wont be checked and they will appear on output list.
As a result I'll get list of all users with MANAGER level that are eather MANAGER on project or not in the project at all.

Is that correct??