Some JIRA instances have a "Participant" field from other plugin's that is different from the "Request Participant" field that JIRA ServiceDesk uses. Can you confirm that you added the customers as a "Request Participant" to the issue?
The second thing to check would be to ensure that your customer's email/account is listed in your "Customers" role on your project, or that they are part of a group that is listed within that role.
If these two things are set correctly, then the customer should be able to log in to the customer portal to make a comment, or reply to an email to add a comment.
I have multiple projects that use variations of the same base workflow. The variations depend on the requirements of the project or issue type. The variations mostly come in the form of new statuses ...
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