On 'Add an Application User Passing Arguments on the Command Line' section on the root README.md it instructs to use: add-user.sh -a -u 'quickstartUser' -p 'quickstartPwd1!' -g 'guest'

The -g switch seems to be changed to -ro on newer EAP versions so the right command instruction should be

add-user.sh -a -u 'quickstartUser' -p 'quickstartPwd1!' -ro 'guest'