How do you self-onboard users who never chose your product?

We’re facing an onboarding challenge with an invite-based B2B platform

Our customers invite external partners who need to accept an invitation, create a profile and configure their account before they can begin collaborating

Today, this often requires a demo involving both companies

That works when a customer has a few partners, but becomes difficult when they need to onboard dozens or hundreds

The main questions:

How do you create successful self-service onboarding for users invited by another company?

Has anyone solved this at a larger scale?