As mentioned in this article, you can invite a patient at any time; before, during and after a treatment episode. In order to do this you'll navigate to either their Facesheet or if they haven't admitted yet, you'll want to go to their initial guest information page and click the "patient portal" button.
That will open a modal where you can invite the patient or contact(s).
Once you click "Invite" for that particular person you will get an access code. That code can be read to a patient over the phone if you'd like. ZenCharts will also send them a secure email and/or text message with the code they will need to sign up.