The main concept is Multitenancy.
Going mainstream is a big challenge for all platforms that targeted enterprises with a multitenant architecture.
The Collaboration across tenants/organizations is a hot topic right now. Businesses that use SaaS software in their tenant realize they need to interact with Users that are supposed to be on the same Platform, but technically are part of another Tenant. In practice these two tenants can be seen as two completely different systems that physically may be unreachable from one to another.
Providers are accommodating for these requirements in different ways.
Identity and Account are two different things. You can still you your Identity (email) and be part of multiple Accounts.
Microsoft chose the very segregated approach (enterprise oriented) where a User (same email/identity)) becomes a Guest in another Account (Organization) by using the Invitations mechanism https://developer.microsoft.com/en-us/graph/docs/api-reference/v1.0/resources/invitation.
Google got it right in terms of UX. You signin once and you can see all the files that are shared with you in Docs/Drive. I don’t really their design here.