I've setup Teams on both my mobile and desktop devices. On the former, I'm able to add multiple accounts / organizations, however, on the latter, I don't seem to be able to. On mobile, I go to settings and then add account.
This isn't about adding multiple teams but about adding multiple (and different) organizations / companies using different email addresses.
Adding multiple accounts in only supported on the mobile client for the time being. On the desktop one, you are bound to a single account and switching between guest tenants. As a workaround, you can create a "wrapper" for the Teams app and run it with multiple instances.