Request participants are treated like customers.

 
Customers do not have access to Issues through the Jira Service Desk (SD) unless they are an agent on that SD. We ask agents to go through the portal for issues outside of the SD they are an agent on.
In the upper right corner (assuming you are already logged in), click on the hamburger icon and select service desk portal.

From there, on the upper right side, click on request.

Then filter by Where I am a participant. This should show all of the public information on the issue.

It should be noted that once you have selected the request to view, adding participants can be accomplished by selecting the share link.