Onedrive collaboration between multiple tenants

There are ways to get this done with B2B. This involves using our APIs and using scripts to bulk onboard people from the other organization. And enabling self service sign-up for new users who join the other company.

Scripts: https://docs.microsoft.com/en-us/azure/active-directory/active-directory-b2b-code-samples 

Self service sign-up portal: https://docs.microsoft.com/en-us/azure/active-directory/active-directory-b2b-self-service-portal

You can see a demo of the self service portal in my talk: https://aka.ms/b2bmechanics 

Then, run a PowerShell script to make these users visible in SPO:

The ability to search for existing guest users in the SharePoint Online people picker is OFF by default to match legacy behavior. You can enable this using the setting 'ShowPeoplePickerSuggestionsForGuestUsers' at the tenant and site collection level. This can be set using the Set-SPOTenant and Set-SPOSite cmdlets, which allow members to search all existing guest users in the directory. Changes in the tenant scope do not affect already provisioned SPO sites.