You can try creating an account with a throw-away gmail address for testing this.
If you enable the “Create or Sync Discourse Users on Login” setting on the WP Discourse SSO Provider tab, Discourse accounts should be automatically created for WordPress users when the user logs into WordPress.
If you are adding users to Discourse groups via Zapier, the request will fail if the user doesn’t exist on Discourse yet. The best way to avoid issue with this would be to add users to groups with SSO. Setting this up is somewhat technical though: Managing Discourse group membership with WP Discourse.