This feature is available in all editions.
You can invite guest collaborators (non-VersionOne members) to participate in any conversation. To do so, simply type their email address in the Mention field. The guest collaborator will receive an email message with a link to register. After registering, they can participate in the conversation stream.
Guest Collaborators do not have access to any projects or assets in the system. They only have access to conversations where they're mentioned. If they're part of a conversation where someone has mentioned a secured asset, the guest collaborator will see the conversation, but not the mentioned asset. Note that Member and System Administrators can promote a guest collaborator to a fully licensed member. They can also deactivate guest collaborators.