Voluntaryism doesn't preclude private property. Again, the only moral way for people to engage with each other is by consent, which requires the ability to exclude (freedom of association). Coops, so long as they're consensual, are by definition voluntary.

Working for someone isn't inherently bad or unjust; you agree to provide your time and perform tasks in exchange for something that's worth more than your time doing something else. A coop is no more required to employ you or let you use their cooperatively owned resources than a traditional boss is.