I haven't tried creating a user from the Emergency Maintenance shell, so don't know if it is possible. But the basic command is as follows:-
cf adminuser add name=<username> password=<password> directory=<home_directory_name> role=admin
So, for a user called "joe" with a password of "password" I would enter the command:-
cf adminuser add name=joe password=password directory=/home/joe role=admin
Hope that helps.