Yeah, this is a line I wrote and could probably improve the clarity on. It's worth distinguishing the Tesseral concept of a User from the sense in which we might colloquially refer to a user. Some other people call the equivalent of a Tesseral User a Member or something similar.
An individual human being who wants to log in can be represented by multiple Users in Tesseral, each of which belongs to exactly one Organization.
That is, there's support for a given person with a given email address to participate in different workspaces, but each workspace will have a different instance of a User.