Short answer
HubSpot's KB: the main account where you create the organization must have an Enterprise subscription, and you must be a Super Admin in both the Enterprise account and each account you add. Find it under Settings > Organization Management > Multi-Account. In my demo portal Create Organization is greyed out, with an Eligibility Rules link beside it.
1. The page
My demo portal:
2. Not the same as
- Teams within one account. See permission sets.
- Copying assets between accounts, which is the separate Asset Copying page.