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"
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)