Just to be clear, while the steps below *work* at this time, it apparently is an unsupported and inappropriate workaround and there could be risks (financial or otherwise) for BIM 360 Docs customers who invite free users into their projects. You should only invite:
BIM 360 Docs users for which you have paid for and applied an entitlement
BIM 360 Design (C4R) users who have purchased their ownentitlement
I expect this workaround may eventually be disabled by Autodesk? You can view if you have exceeded your purchased package of BIM 360 Docs entitlements in the relevant Autodesk Manage pages. For example, it might show 16 of 10 licenses used in red? This is where I think there is potential for Autodesk to back charge users who abuse this current state of affairs.
My advice: check directly with Autodesk about who you can and cannot share your BIM 360 projects with…and proceed accordingly.
You are a paying BIM 360 Docs user and would like to invite a non-paying user to collaborate inside a BIM 360 Docs workspace. Perhaps you just want them to be able to upload and download files, but not Collaborate in Revit…
They need to create a free BIM 360 Docs account, and then get invited to your workspace.