Warren, in addition Mick's answer, the assignee and reporter fields do not support the 'contains:~' operator, as they're not free text fields.
You might want to try creating a custom free text field to be filled with the email addresses of the issue participants, and that would be searchable.
Hope it helps.
Add a scripted field (and call it any name) (https://marketplace.atlassian.com/plugins/com.onresolve.jira.groovy.groovyrunner) and add this code:
Then search using this new field.
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