It works the other way; you can't _not_ have multiple screen names. Each workspace (which is the new, not as good name for a slack team) has its own unique set

It works the other way; you can't _not_ have multiple screen names. Each workspace (which is the new, not as good name for a slack team) has its own unique set of users. Every workspace whose slack you join means you need to create a new user for that workspace.