@████████ as far as I'm aware it is not possible under any official licensing model to include public (non-registered) users as part of an approval process. I was unable to attend Empower this year so I'm not sure if that is being worked on or changing. 

There are some creative solutions out there to get around this limitation. Some people have used the "Workflow Service" task as part of a business process to "pause" their primary process in place, while Laserfiche Workflow sends an email with a URL for a secondary public process that includes a GUID to your target user.

The same workflow then waits for this public form to be completed/submitted using the GUID. After the submission is completed, the Laserfiche Workflow releases the original primary process with data from the secondary "public" process (eg. was it approved or rejected?).

It is convoluted, and for advanced users (as it relies on Workflow), but it is a solution that works somewhat reliably.