This feature request is in the works!
The team is hard at work to ensure it is implemented in a fashion that will meet your expectations.
We know you are patiently waiting for this feature. This feature is being implemented in stages. Fast tenant switching is currently in testing both internally and with early customers. We expect to release to the public in the Fall. I’ll post an update once it’s rolling out.
Full multi-tenant, multi-user support will be added after fast tenant switching.
~AlexRyan Calafato commented
I am a consulting agency and we use Office365 for our own needs. I have a client, that is also an Office365 customer. We're collaborating on a long-term project. 5 people from my agency will be working on this project, including me. My client has given me an account in their directory. The client wants to use Teams to manage the project. I'm the only person from my agency who can access Teams for this project, and we're still managing our out Team space for collaboration. The goal is that everyone that needs to can access the Team space for the project, using the desktop app, and with a single logon.