Adding the iip user in OS/X

I'm having trouble installing iip on OS/x.  Before running 'make
install', the docs state that the user iip must exist.  I ran 'make
install' anyway to see if it would create the user for me, and it
errored because of the missing user.  The docs say there's a specific
makefile for the Mac, but I couldn't find any file that seemed to fit
the bill.

OS/x does not come with the adduser or useradd commands; in fact,
searching the man pages doesn't come up with any tool for adding
users.  Any hints?

I know I can add users through the GUI, but it doesn't seem right for
the iip user.  I assume the iip user is not a user that I will log in
with, but one of these system users that a daemon might use.  I don't
fully understand why these users exist, but I know the passwd file has
a completely different set of users than the gui.  The gui lists the
one user, not even the root user.  Then the passwd file lists several
system users, but not the user that the gui lists.

My last attempt was to manually add an iip user to the passwd file,
and create the /home/iip directory.  Then the wall I hit was when I
tried to chown the /home/iip directory to the ipp user.  Despite the
entry in the passwd file, the user 'iip' did not exist (according to
chown), so I could not change ownership of the home directory.