Regardless of a user's type - i.e. whether they are a workspace collaborator or have been added directly as a guest - and the combination of apps they have been granted access to, the method for registration and login is the same.
If you have added a user as a workspace collaborator or manually as a guest they will receive a welcome email which should direct them to sign up for an account at your workspace URL /register. If you added them via connecting a user table, or if they didn't receive an email you can send them directly to the URL to register.
Note: Workspaces currently do not support open signup - a user's email address will not be accepted on /register if they haven't already been added to your workspace/an app.
Once registered, users can login via your workspace URL /login. The workspace login page will be branded with your logo (if you have added one). When a workspace user logs in, they will land in the first app they have access to, but can easily switch between apps using the app bar.