JIRA Service Desk have the "Public Sign Up" feature that will let unregistered user that accessing the portal to be able to sign up and create an account, this also includes for unregistered user that is sent request through email. When a unregistered user sent an email to Service Desk email channel, the email will be converted to a request and an invitation link will sent out to the sender to notify them.
But, at some situation, the sender of the email will forward the created request notification to another user that will be take part on that request, and there is a possibility that the user that received the forward email is an unregistered user as well. Then, that user will reply to Service Desk email channel with the same subject as the previous notification with intend to update/comment on the ticket. Since this user is not registered yet, Service Desk will process the email but did not do aything with it as the subject is the same and it is from an unregistered user email address.
Hence, it will be great if JIRA Service Desk Public Sign Up will cover those unregistered user that have an intend to comment on an existing ticket. Making the user does not need to access the portal and sign up just to do a simple comment, also it will automatically include the user as the participant of that request.
It could perhaps be good to add some sort of approve-mechanism, if an user wants to comment on an issue, but aren't allowed because the user user is not in request participant list. As if it is possible, it would make the customer to be added to the request which could lead to a security issues.