User management is disabled by default. After you start pulpTunes, right-click on the orange in the task tray (menu bar if you're in Mac), and select "user management". Then tick "Enable User Management" and fill in the credentials for the admin user. Then the pulpTunes webpage will show a new link on top to log-in. After logging-in as admin, you'll have yet another link "user management" that'll give you access to the groups and users management section.
Cheers