New users can be created two ways. They can be set up by the knowledgebase administrator, or users can register themselves.
By default, KBPublisher creates registered users with no designated role. You can change this so that new users are created with a specific role by:
First, ensure that the role exists. If it doesn't, create it first
From the Settings menu choose the Public Area -> Common tab
Go to the Security / Privacy / Registering section
Select the appropriate Role for new registered users from the drop-down list of roles
All users who register after this are created with the new default role.
You must have administrator access to change the settings.