Multi Tenant vs Single Tenant for teams deployment

For a single organisation, a single tenant model provides the best user experience and single point of administration, however there is significant work involved in migrating existing tenants, depending on the maturity of those tenancies.

Collaboration between people hosted in separate tenancies is possible, with Microsoft Teams being a great example of an app that is suited to such scenarios. People from tenancy "A" can be "guests" within tenancy "B", and granted access to resources such as a Microsoft Teams team, and visa-versa.

An important point is that a guest does not have the same experience/rights in Microsoft Teams as a normal user when accessing another tenancy as a guest (they will not see their own calendar or OneDrive). This does not just relate to Microsoft Teams but other apps in Microsoft 365 (for example Microsoft Stream videos cannot be viewed by guests ATM).

It is also worth noting that within the Microsoft Teams app, the person needs to switch tenancy context when moving from Tenancy A to Tenancy B.

Here is a useful resource on guest access in https://docs.microsoft.com/en-us/microsoftteams/guest-access

Here is a useful resource on Tenancy migrations https://docs.microsoft.com/en-us/office365/enterprise/microsoft-cloud-it-architecture-resources#architecture-approaches-for-microsoft-cloud-tenant-to-tenant-migrations