Thanks for reading the tutorial. Page 36 shows you how to start the workflow in the Alfresco Explorer (/alfresco) web client. You just click the "Start Advanced Workflow" link and select your workflow from the list.
If you are using Alfresco Share you need to read the 2nd edition of the tutorial instead of the first edition, which only covers the Explorer web client.
It does not matter which users you create, how many you create, or what their user names are. But you do need to make sure you add them to the engineering and marketing groups as specified in the tutorial.