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Requested participants of a issue which are only Jira Servicedesk portal customers cannot being mentioned, unless they have left a comment in the ticket, then they will become a commenter on the issue.
People already involved in the issue, like the reporter or a commenter, will be listed first in the user list so you can select them faster. Note that the users you mention will be notified once you save the issue description or comment.
User mention via comment using autocomplete
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