Default Admin User
Default "admin" user (user_id 1) has permission to see all pages within the application.
Default Guest User
If a user has not logged in, the user is considered a guest user and has same permission as the "guest" user (user_id -1001). If you want to allow anonymous access, you can do so by granting permission to the "guest" user.
To control what pages a user can see, you can modify the permission settings on the user account. Or you can put the user in a group and modify the permission settings on the group.
New in version 2.7.4. On User Detail page, a "Delete User" is available when the user being deleted is not a default user and not the same as the login user.
Users deleted will be stored in an archive table hidden from the web interface. Permissions, group mapping, issue subscriptions, etc will be deleted.