Unable to add a non-service-desk-customer as Participants

Summary

Trying to add a non-service-desk-customer into the participant list even though the Service Desk is public and Anyone can sign up for a customer account on my Customer Portal option is enabled

Steps to Reproduce

1. Raise a request into the Customer Portal
2. Click on the +Add People button to add a participants
3. Insert an email that has not been registered as Service Desk customer

Expected Results

Service Desk will send a notification email that the user should sign up to view the request in Service Desk Customer Portal

Actual Results

Could not find an account for user@email.com, or they cannot be added to this request

message is shown, and the requested participant(s) not added

Notes

Is this a bug or expected behavior? If this is a Feature Request, then it's gonna be:

As an agent or customer, I would like to add participants who is not already in the Service Desk Customer list

Workaround

Invite user first before adding them as participant