There is not such thing as "user" in PmWiki - everybody can be user
You just need to know passwords to edit some pages or change attributes of pages
Default password to edit in groups "Users" is "User3"
See
http://www.3niti.org/pmwiki.php?n=Users.Users where described approach for creating new pages - simply use URL in form
http://3niti.org/pmwiki.php?n=Users.You ... ction=edit (don't forget to change "YourName" to your name)