If you do not need authentication or authorization at all (for example, you're only running npm On-Site inside your firewalled, private network), you can configure npm On-Site to accept whatever credentials users log in with, and allow everyone access to everything.
In order to do so, visit npm On-Site's admin console, and choose the Open authentication strategy.
The npm CLI will still require that you log in in order to publish
packages, but you can just log in with any credentials (although the
username you input will be inserted into the package document - for
example in the maintainers field).