The case
Two companies (for example, parent and affiliate) have their own Microsoft tenants. Both companies are big, let's say more than 10000 users each. Everyday companies hire and fire employees.
The first company (parent) has an Internal website on SharePoint Online. Employees of the second company (child) have to get access to this website. Someone else shouldn't have rights.
Standard approach of Azure B2B "invite each user" is not a suitable option. You (or someone else) can't (it's possible, but I assume you do not want this) manually send an individual invitation for each new user of child company.
Is there another way? Yes, but it's not perfect.
