_mbsetupuser and Giving local admin rights

All - using the following script we are giving the user local admin rights. It is set for Login, once per user per computer.

#!/bin/bash

loggedInUser=$(stat -f '%Su' /dev/console)
evaluate=$( /usr/sbin/dseditgroup -o checkmember -m $loggedInUser admin )

echo $evaluate

case $evaluate in
    *yes* ) echo User is already an admin on this mac;;
    *no* ) /usr/sbin/dseditgroup -o edit -a $loggedInUser -t user admin;;
    * ) echo Something went wrong;;
esac

We're seeing an issue, intermittently, where the user isnt getting the rights. In the logs, it looks like its triggered when _mbsetupuser is logged in, and not the actual user we are using... What is _mbsetupuser, and does anyone know why this might be happening?

Thanks!
Ben