You technically do not need an add-on to do this with Service Desk today. If you are a JSD Agent, you can visit the customer portal view of this issue, and select the "Share" link on that page. From here you can enter in any email address and that will send a notification to that email address to inform them that they have been added as a requested participant.
However this adds this email address to the system as a requested participant, which technically is no longer an external account in the eyes of Service Desk.
But if you're heart is set on a plugin to handle this external notifications, or you don't want to add the user as a requested participant, I would suggest something like Jirassimo. I have not personally used it, but I understand it to have these kinds of notification features that I believe will leave the external account as just that, external.
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